Annual report pursuant to Section 13 and 15(d)

Basis of presentation (Details)

v3.8.0.1
Basis of presentation (Details)
$ in Thousands
3 Months Ended 9 Months Ended 12 Months Ended
Jul. 15, 2016
segment
Mar. 31, 2016
USD ($)
Sep. 30, 2016
segment
Dec. 31, 2017
USD ($)
segment
Dec. 31, 2016
USD ($)
Interim Consolidated Financial Information          
Number of operating segments | segment 2   1 2  
Deferred income taxes       $ 39,269 $ 54,563
Litigation proceeds   $ 10,050   1,275 $ 10,050
Accounting Standards Update 2015-17 [Member]          
Interim Consolidated Financial Information          
Deferred income taxes       $ 5,726  
Interest Rate Swap Effective 31 December 2015 through 29 March 2018          
Interim Consolidated Financial Information          
Fixed interest rate (as a percent)       6.916%  
Interest Rate Swap Effective 31 December 2015 through 29 March 2018 | London Interbank Offered Rate (LIBOR)          
Interim Consolidated Financial Information          
Interest rate added to variable rate (as a percent)       3.00%  
LIBOR floor (as a percent)       1.00%  
Interest Rate Swap Effective 29 March 2018 through 31 March 2020          
Interim Consolidated Financial Information          
Fixed interest rate (as a percent)       6.105%  
Interest Rate Swap Effective 29 March 2018 through 31 March 2020 | London Interbank Offered Rate (LIBOR)          
Interim Consolidated Financial Information          
Interest rate added to variable rate (as a percent)       3.00%  
LIBOR floor (as a percent)       1.00%  
Interest Rate Swap Effective 31 March 2020 through 30 June 2021          
Interim Consolidated Financial Information          
Fixed interest rate (as a percent)       7.168%  
Interest Rate Swap Effective 31 March 2020 through 30 June 2021 | London Interbank Offered Rate (LIBOR)          
Interim Consolidated Financial Information          
Interest rate added to variable rate (as a percent)       3.00%  
LIBOR floor (as a percent)       1.00%