Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Other comprehensive income (loss):
Other comprehensive income, before tax	$ (45,516)	$ 15,892	$ (70,704)	$ 90,716
Other comprehensive income (loss):
Other comprehensive income, tax effect	8,599	(3,856)	10,944	(14,469)
Net of Tax
Balance, beginning of period			3,862,633
Other comprehensive income (loss):
Total other comprehensive (loss) income	(36,917)	12,036	(59,760)	76,247
Balance, end of period	4,167,984	3,746,544	4,167,984	3,746,544
Accumulated Other Comprehensive Income (Loss)
Before Tax
AOCI before tax, beginning of period	143,136	144,225	168,324	69,401
Other comprehensive income (loss):
AOCI before tax, end of period	97,620	160,117	97,620	160,117
Tax Effect
AOCI tax effect, beginning of period	(27,705)	(23,827)	(30,050)	(13,214)
Other comprehensive income (loss):
AOCI tax effect, end of period	(19,106)	(27,683)	(19,106)	(27,683)
Net of Tax
Balance, beginning of period	115,431	120,398	138,274	56,187
Other comprehensive income (loss):
Total other comprehensive (loss) income	(36,917)	12,036	(59,760)	76,247
Balance, end of period	78,514	132,434	78,514	132,434
Accumulated Net Investment Gains (Losses) On Investments
Other comprehensive income (loss):
Unrealized holding (losses) gains arising during the period, before tax	(45,295)	16,159	(70,095)	92,849
Less: Reclassification adjustment for (losses) gains included in net income, before tax	(221)	(267)	(609)	(2,133)
Other comprehensive income, before tax	(45,516)	15,892	(70,704)	90,716
Other comprehensive income (loss):
Unrealized holding (losses) gains arising during the period, tax effect	8,548	(3,910)	10,923	(14,851)
Less: Reclassification adjustment for gains (losses) included in net income, tax effect	51	54	21	382
Other comprehensive income, tax effect	8,599	(3,856)	10,944	(14,469)
Other comprehensive income (loss):
Unrealized holding (losses) gains arising during the period, net of tax	(36,747)	12,249	(59,172)	77,998
Less: Reclassification adjustment for gains (losses) included in net income, net of tax	(170)	(213)	(588)	(1,751)
Total other comprehensive (loss) income	$ (36,917)	$ 12,036	$ (59,760)	$ 76,247

AOCI Attributable To Parent, Before Tax [Roll Forward]

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