Other Comprehensive Income (Loss) - Other comprehensive income (loss) (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Other comprehensive income (loss), before tax amount
Other comprehensive income (loss), before tax amount		$ (431)	$ (96,262)	$ (79,156)
Other comprehensive income (loss), tax (expense) benefit
Other comprehensive income (loss), tax (expense) benefit		532	35,062	(33,370)
Other comprehensive income (loss):
Total other comprehensive income (loss), net of tax		101	(61,200)	(112,526)
Deferred tax asset valuation allowance released					$ 63,000
Deferred tax asset valuation allowance reclassified from AOCI	$ 63,000
Foreign Currency Translation Adjustment
Other comprehensive income (loss), before tax amount
Other comprehensive income (loss), before reclassifications, before tax		1,027
Other comprehensive income (loss):
Other comprehensive income (loss), before reclassifications, net of tax		1,027
Unrealized/Recognized Gains (Losses)
Other comprehensive income (loss), before tax amount
Other comprehensive income (loss), before reclassifications, before tax		9,671	3,050	20,205
Amounts reclassified from accumulated other comprehensive income (loss), before tax		(11,129)	(99,312)	(99,361)
Other comprehensive income (loss), tax (expense) benefit
Other comprehensive income (loss), before reclassifications, tax (expense) benefit		(3,525)	(1,111)	(7,476)
Amounts reclassified from accumulated other comprehensive income (loss), tax (expense) benefit		4,057	36,173	(25,894)
Other comprehensive income (loss):
Other comprehensive income (loss), before reclassifications, net of tax		6,146	1,939	12,729
Amounts reclassified from accumulated other comprehensive income (loss), net of tax		$ (7,072)	$ (63,139)	$ (125,255)