Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Other comprehensive income (loss):
Other comprehensive income (loss), before tax	$ 6,911	$ 19,073
Other comprehensive income (loss):
Other comprehensive income (loss), tax (expense) benefit	(2,061)	(5,714)
Net of Tax
Stockholders equity, beginning of period	1,343,773	1,119,241
Other comprehensive income (loss):
Other comprehensive income (loss), net of tax	4,850	13,359
Stockholders equity, end of period	1,412,752
Accumulated Other Comprehensive Income (Loss)
Before Tax
AOCI before tax, beginning of period	(14,436)	661
Other comprehensive income (loss):
AOCI before tax, end of period	(7,525)	19,734
Tax Effect
AOCI tax effect, beginning of period	2,181	(760)
Other comprehensive income (loss):
AOCI tax effect, end of period	120	(6,474)
Net of Tax
Stockholders equity, beginning of period	(12,255)	(99)
Other comprehensive income (loss):
Stockholders equity, end of period	(7,405)	13,260
Accumulated Net Investment Gains (Losses) On Investments
Other comprehensive income (loss):
Unrealized holding gains arising during the period, before tax	7,566	19,551
Less: Reclassification adjustment for gains included in net income	(655)	(478)
Other comprehensive income (loss), before tax	6,911	19,073
Other comprehensive income (loss):
Unrealized holding gains arising during the period, tax (expense) benefit	(2,289)	(5,845)
Less: Reclassification adjustment for losses included in net income	228	131
Other comprehensive income (loss), tax (expense) benefit	(2,061)	(5,714)
Other comprehensive income (loss):
Unrealized holding gains arising during the period. net of tax	5,277	13,706
Less: Reclassification adjustment for losses included in net income	(427)	(347)
Other comprehensive income (loss), net of tax	$ 4,850	$ 13,359