Application of Recent Accounting Pronouncements (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jan. 01, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total stockholder’s equity	$ 2,039,996	$ 2,207,290		$ 2,449,589	$ 2,012,321
Retained earnings	1,132,136			1,051,792
Cash flows from operating	261,940	481,107
Cash flows from investing	(243,328)	(708,699)
Cash flows from financing	$ 52,283	226,110
Expected life of customer contract	10 years
Retained earnings
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total stockholder’s equity	$ 1,132,136	853,473			$ 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		21,388
Cash flows from financing		$ 21,388