STATEMENTS OF COMPREHENSIVE INCOME (LOSS) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Statement Of Income And Comprehensive Income [Abstract]
Net Income (Loss)	$ 11,965	$ 24,816
Net unrealized gains (losses) on available-for-sale securities
Net unrealized holding gains (losses) arising during the period	(44,193)	5,996
Reclassification adjustment for (gains) losses included in net income (loss)	(11,450)	532
Total net unrealized gains (losses) on available-for-sale securities	(55,643)	6,528
Net unrealized gains (losses) on cash flow hedges
Net unrealized gains (losses) on cash flow hedges arising during the period	1,792	161
Reclassification adjustment for (gains) losses included in net income (loss)	(1,303)	247
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, before Tax, Total	489	408
Net unrealized other-than-temporary impairments on securities
Change in previously recognized unrealized other-than-temporary impairments	(6)	(77)
Net unrealized other-than-temporary impairments on securities	(6)	(77)
Adjustments
Value of business acquired	8,784	(871)
Total other comprehensive income (loss), net of taxes	(46,376)	5,988
Comprehensive Income (Loss)	$ (34,411)	$ 30,804