Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Other comprehensive income (loss):
Other comprehensive gains (losses), before tax	$ 98,923	$ 102,677
Other comprehensive income (loss):
Other comprehensive gains (losses), tax effect	(16,836)	(17,497)
Net of Tax
Stockholders equity, beginning of period	2,984,845	2,365,717
Other comprehensive income (loss):
Total other comprehensive income (loss)	82,087	85,180	$ (25,741)
Stockholders equity, end of period	3,862,633	2,984,845	2,365,717
Accumulated Other Comprehensive Income (Loss)
Before Tax
AOCI before tax, beginning of period	69,401	(33,276)
Other comprehensive income (loss):
AOCI before tax, end of period	168,324	69,401	(33,276)
Tax Effect
AOCI tax effect, beginning of period	(13,214)	4,283
Other comprehensive income (loss):
AOCI tax effect, end of period	(30,050)	(13,214)	4,283
Net of Tax
Stockholders equity, beginning of period	56,187	(28,993)	(3,252)
Other comprehensive income (loss):
Total other comprehensive income (loss)	82,087	85,180	(25,741)
Stockholders equity, end of period	138,274	56,187	$ (28,993)
Accumulated Net Investment Gain (Loss) Attributable to Parent
Other comprehensive income (loss):
Unrealized holding gains (losses) arising during the period, before tax	101,620	105,906
Less: Reclassification adjustment for gains included in net income, before tax	(2,697)	(3,229)
Other comprehensive gains (losses), before tax	98,923	102,677
Other comprehensive income (loss):
Unrealized holding gains (losses) arising during the period, tax effect	(17,286)	(17,995)
Less: Reclassification adjustment for gains included in net income, tax effect	450	498
Other comprehensive gains (losses), tax effect	(16,836)	(17,497)
Other comprehensive income (loss):
Unrealized holding gains (losses) arising during the period, net of tax	84,334	87,911
Less: Reclassification adjustment for gains included in net income, net of tax	(2,247)	(2,731)
Total other comprehensive income (loss)	$ 82,087	$ 85,180