STATEMENTS OF COMPREHENSIVE INCOME (LOSS) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement Of Income And Comprehensive Income [Abstract]
Net Income (Loss)	$ 99,405	$ (20,527)	$ 13,658
Net unrealized gains (losses) on available-for-sale securities
Net unrealized holding gains (losses) arising during the period	29,511	8,993	(67,572)
Reclassification adjustment for (gains) losses included in net income (loss)	(1,440)	3	(484)
Total net unrealized gains (losses) on available-for-sale securities	28,071	8,996	(68,056)
Net unrealized gains (losses) on cash flow hedges
Net unrealized gains (losses) on cash flow hedges arising during the period	(806)	(6,288)	1,807
Reclassification adjustment for (gains) losses included in net income (loss)	1,100	861	99
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, before Tax, Total	294	(5,427)	1,906
Net unrealized other-than-temporary impairments on securities
Change in previously recognized unrealized other-than temporary impairments	923	(112)	(741)
Reclassification adjustment for other-than-temporary impairments included in net income (loss)		121	109
Net unrealized other-than-temporary impairments on securities	923	9	(632)
Adjustments
Value of business acquired	(4,139)	(4,819)	14,131
Total other comprehensive income (loss), net of taxes	25,149	(1,241)	(52,651)
Comprehensive Income (Loss)	$ 124,554	$ (21,768)	$ (38,993)