Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Other comprehensive income (loss):
Other comprehensive income, before tax	$ 12,119	$ 15,751	$ 19,030	$ 34,824
Other comprehensive income (loss):
Other comprehensive income, tax effect	(3,649)	(5,049)	(5,710)	(10,763)
Net of Tax
Stockholders equity, beginning of period			1,343,773	1,119,241	$ 1,119,241
Other comprehensive income (loss):
Total other comprehensive income	8,470	10,702	13,320	24,061	(12,156)
Stockholders equity, end of period	1,497,897		1,497,897		1,343,773
Accumulated Other Comprehensive Income (Loss)
Before Tax
AOCI before tax, beginning of period	(7,525)	19,734	(14,436)	661	661
Other comprehensive income (loss):
AOCI before tax, end of period	4,594	35,485	4,594	35,485	(14,436)
Tax Effect
AOCI tax effect, beginning of period	120	(6,474)	2,181	(760)	(760)
Other comprehensive income (loss):
AOCI tax effect, end of period	(3,529)	(11,523)	(3,529)	(11,523)	2,181
Net of Tax
Stockholders equity, beginning of period	(7,405)	13,260	(12,255)	(99)	(99)
Other comprehensive income (loss):
Total other comprehensive income			13,320		(12,156)
Stockholders equity, end of period	1,065	23,962	1,065	23,962	$ (12,255)
Accumulated Net Investment Gains (Losses) On Investments
Other comprehensive income (loss):
Unrealized holding gains arising during the period, before tax	12,663	16,334	20,229	35,885
Less: Reclassification adjustment for gains included in net income, before tax	(544)	(583)	(1,199)	(1,061)
Other comprehensive income, before tax	12,119	15,751	19,030	34,824
Other comprehensive income (loss):
Unrealized holding gains arising during the period, tax effect	(3,833)	(5,185)	(6,122)	(11,030)
Less: Reclassification adjustment for gains included in net income, tax effect	184	136	412	267
Other comprehensive income, tax effect	(3,649)	(5,049)	(5,710)	(10,763)
Other comprehensive income (loss):
Unrealized holding gains arising during the period, net of tax	8,830	11,149	14,107	24,855
Less: Reclassification adjustment for gains included in net income, net of tax	(360)	(447)	(787)	(794)
Total other comprehensive income	$ 8,470	$ 10,702	$ 13,320	$ 24,061