Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Other comprehensive income (loss):
Other comprehensive income (loss), before tax	$ 46,317	$ (33,943)
Other comprehensive income (loss):
Other comprehensive (income) loss, tax effect	(7,951)	5,193
Net of Tax
Stockholders equity, beginning of period	2,365,717	1,940,436
Other comprehensive income (loss):
Total other comprehensive income (loss)	38,366	(28,750)
Stockholders equity, end of period	2,527,803	1,995,290
Accumulated Other Comprehensive Income (Loss)
Before Tax
AOCI before tax, beginning of period	(33,276)	(1,849)
Other comprehensive income (loss):
AOCI before tax, end of period	13,041	(35,792)
Tax Effect
AOCI tax effect, beginning of period	4,283	(1,403)
Other comprehensive income (loss):
AOCI tax effect, end of period	(3,668)	3,790
Net of Tax
Stockholders equity, beginning of period	(28,993)	(3,252)
Other comprehensive income (loss):
Total other comprehensive income (loss)	38,366	(28,750)
Stockholders equity, end of period	9,373	(32,002)
Accumulated Net Investment Gains (Losses) On Investments
Other comprehensive income (loss):
Unrealized holding gains (losses) arising during the period, before tax	46,977	(33,746)
Less: Reclassification adjustment for gains included in net income, before tax	(660)	(197)
Other comprehensive income (loss), before tax	46,317	(33,943)
Other comprehensive income (loss):
Unrealized holding (gains) losses arising during the period, tax effect	(8,071)	5,120
Less: Reclassification adjustment for gains included in net income, tax effect	120	73
Other comprehensive (income) loss, tax effect	(7,951)	5,193
Other comprehensive income (loss):
Unrealized holding gains (losses) arising during the period, net of tax	38,906	(28,626)
Less: Reclassification adjustment for gains included in net income, net of tax	(540)	(124)
Total other comprehensive income (loss)	$ 38,366	$ (28,750)