Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended	12 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Other comprehensive income (loss):
Other comprehensive income (loss), before tax	$ (8,664)	$ 12,119	$ (42,607)	$ 19,030
Other comprehensive income (loss):
Other comprehensive (income) loss, tax effect	1,418	(3,649)	6,611	(5,710)
Net of Tax
Stockholders equity, beginning of period			1,940,436	1,343,773	$ 1,343,773
Other comprehensive income (loss):
Total other comprehensive (loss) income	(7,246)	8,470	(35,996)	13,320	8,068
Stockholders equity, end of period	2,103,571		2,103,571		1,940,436
Accumulated Other Comprehensive Income (Loss)
Before Tax
AOCI before tax, beginning of period	(35,792)	(7,525)	(1,849)	(14,436)	(14,436)
Other comprehensive income (loss):
AOCI before tax, end of period	(44,456)	4,594	(44,456)	4,594	(1,849)
Tax Effect
AOCI tax effect, beginning of period	3,790	120	(1,403)	2,181	2,181
Other comprehensive income (loss):
AOCI tax effect, end of period	5,208	(3,529)	5,208	(3,529)	(1,403)
Net of Tax
Stockholders equity, beginning of period	(32,002)	(7,405)	(3,252)	(12,255)	(12,255)
Other comprehensive income (loss):
Total other comprehensive (loss) income			(35,996)		8,068
Stockholders equity, end of period	(39,248)	1,065	(39,248)	1,065	$ (3,252)
Accumulated Net Investment Gains (Losses) On Investments
Other comprehensive income (loss):
Unrealized holding gains (losses) arising during the period, before tax	(8,225)	12,663	(41,971)	20,229
Less: Reclassification adjustment for gains included in net income, before tax	(439)	(544)	(636)	(1,199)
Other comprehensive income (loss), before tax	(8,664)	12,119	(42,607)	19,030
Other comprehensive income (loss):
Unrealized holding (gains) losses arising during the period, tax effect	1,325	(3,833)	6,445	(6,122)
Less: Reclassification adjustment for gains included in net income, tax effect	93	184	166	412
Other comprehensive (income) loss, tax effect	1,418	(3,649)	6,611	(5,710)
Other comprehensive income (loss):
Unrealized holding gains (losses) arising during the period, net of tax	(6,900)	8,830	(35,526)	14,107
Less: Reclassification adjustment for gains included in net income, net of tax	(346)	(360)	(470)	(787)
Total other comprehensive (loss) income	$ (7,246)	$ 8,470	$ (35,996)	$ 13,320

AOCI Attributable To Parent, Before Tax [Roll Forward]

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