Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Other comprehensive income (loss):
Other comprehensive income (loss), before tax	$ 12,587	$ (15,097)
Other comprehensive income (loss):
Other comprehensive income (loss), tax effect	(4,519)	2,941
Net of Tax
Stockholders equity, beginning of period	1,343,773	1,119,241	$ 955,738
Other comprehensive income (loss):
Total other comprehensive income (loss)	8,068	(12,156)	(4,766)
Stockholders equity, end of period	1,940,436	1,343,773	1,119,241
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	(1,849)	(14,436)	661
Tax Effect
AOCI tax effect, beginning of period	2,181	(760)
Other comprehensive income (loss):
AOCI tax effect, end of period	(1,403)	2,181	(760)
Net of Tax
Stockholders equity, beginning of period	(12,255)	(99)	4,667
Other comprehensive income (loss):
Total other comprehensive income (loss)	8,068	(12,156)	(4,766)
Reclassification of certain income tax effects resulting from tax reform	935
Stockholders equity, end of period	(3,252)	(12,255)	$ (99)
Accumulated Net Investment Gain (Loss) Attributable to Parent
Other comprehensive income (loss):
Unrealized holding gains (losses) arising during the period, before tax	14,602	(13,156)
Less: Reclassification adjustment for gains included in net income, before tax	(2,015)	(1,941)
Other comprehensive income (loss), before tax	12,587	(15,097)
Other comprehensive income (loss):
Unrealized holding (gains) losses arising during the period, tax effect	(5,239)	2,376
Less: Reclassification adjustment for gains included in net income, tax effect	720	565
Other comprehensive income (loss), tax effect	(4,519)	2,941
Other comprehensive income (loss):
Unrealized holding gains (losses) arising during the period, net of tax	9,363	(10,780)
Less: Reclassification adjustment for gains included in net income, net of tax	(1,295)	(1,376)
Total other comprehensive income (loss)	$ 8,068	$ (12,156)