Application of Recent Accounting Pronouncements (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Jan. 01, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total stockholder’s equity	$ 2,205,082	$ 2,034,550		$ 2,449,589	$ 2,012,321
Retained earnings	1,109,401			1,051,792
Cash flows from operating	(4,922)	357,163
Cash flows from investing	(76,488)	(505,610)
Cash flows from financing	$ 84,771	143,046
Expected life of customer contract	10 years
Retained earnings
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total stockholder’s equity	$ 1,109,401	863,952			$ 906,122
ASC 606
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total stockholder’s equity				2,482,541
ASC 606 | Retained earnings
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total stockholder’s equity				1,084,744
ASC 606 | Difference between revenue guidance in effect before and after ASC 606
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total stockholder’s equity				32,952
Retained earnings			$ 32,952
ASC 606 | Difference between revenue guidance in effect before and after ASC 606 | Retained earnings
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total stockholder’s equity				$ 32,952
ASU 2016-15, COLI
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Cash flows from operating		(1,680)
Cash flows from investing		1,680
ASU 2016-15, Due to / from parent and affiliate
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Cash flows from operating		2,921
Cash flows from financing		$ (2,921)