Accumulated Other Comprehensive Income (Loss) - Changes (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Pre-tax Amount
Other comprehensive income	$ (2,015)	$ 18,079	$ 113,346	$ 155,222
Income Tax Benefit (Expense)
Other comprehensive income	538	(4,869)	(30,343)	(41,801)
Net of tax
Balance, beginning	2,701,897	2,659,441	2,640,258	2,524,839
Other comprehensive (loss) income	(1,477)	13,210	83,003	113,421
Balance, ending	2,733,934	2,654,558	2,733,934	2,654,558
Accumulated other comprehensive Income (loss)
Pre-tax Amount
Balance	71,911	(43,772)	(43,450)	(180,915)
Balance	69,896	(25,693)	69,896	(25,693)
Income Tax Benefit (Expense)
Balance	(19,180)	11,788	11,701	48,720
Balance	(18,642)	6,919	(18,642)	6,919
Net of tax
Balance, beginning	52,731	(31,984)	(31,749)	(132,195)
Other comprehensive (loss) income	(1,477)	13,210	83,003	113,421
Balance, ending	51,254	(18,774)	51,254	(18,774)
Pension and other benefits, change in Company tax rate
Income Tax Benefit (Expense)
Other comprehensive income			(96)
Net of tax
Other comprehensive (loss) income			(96)
Pension and other benefits, net actuarial losses arising during the period
Pre-tax Amount
Other comprehensive income				(813)
Income Tax Benefit (Expense)
Other comprehensive income				219
Net of tax
Other comprehensive (loss) income				(594)
Pension and other benefits, net change in pension and other benefits
Pre-tax Amount
Other comprehensive income				(813)
Income Tax Benefit (Expense)
Other comprehensive income			(96)	219
Net of tax
Balance, beginning	(28,178)	(28,973)	(28,082)	(28,379)
Other comprehensive (loss) income			(96)	(594)
Balance, ending	(28,178)	(28,973)	(28,178)	(28,973)
Investment securities:
Pre-tax Amount
Unrealized net gains (losses) arising during the period	(1,991)	18,079	113,244	153,443
Reclassification of net losses (gains) to net income	(24)		102	2,592
Other comprehensive income	(2,015)	18,079	113,346	156,035
Income Tax Benefit (Expense)
Unrealized net gains arising during the period	532	(4,869)	(30,220)	(41,322)
Reclassification of net losses (gains) to net income	6		(27)	(698)
Other comprehensive income	538	(4,869)	(30,247)	(42,020)
Net of tax
Balance, beginning	80,909	(3,011)	(3,667)	(103,816)
Unrealized net gains arising during the period	(1,459)	13,210	83,024	112,121
Reclassification of net losses (gains) to net income	(18)		75	1,894
Other comprehensive (loss) income	(1,477)	13,210	83,099	114,015
Balance, ending	$ 79,432	$ 10,199	$ 79,432	$ 10,199