Accumulated Other Comprehensive Income (Loss) - Changes (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Pre-tax Amount
Other comprehensive income	$ 66,282	$ 64,007	$ 115,361	$ 137,143
Income Tax Benefit (Expense)
Other comprehensive income	(17,680)	(17,237)	(30,881)	(36,932)
Net of tax
Balance, beginning	2,664,685	2,613,202	2,640,258	2,524,839
Other comprehensive income	48,602	46,770	84,480	100,211
Balance, ending	2,701,897	2,659,441	2,701,897	2,659,441
Accumulated other comprehensive Income (loss)
Pre-tax Amount
Balance	5,629	(107,779)	(43,450)	(180,915)
Balance	71,911	(43,772)	71,911	(43,772)
Income Tax Benefit (Expense)
Balance	(1,500)	29,025	11,701	48,720
Balance	(19,180)	11,788	(19,180)	11,788
Net of tax
Balance, beginning	4,129	(78,754)	(31,749)	(132,195)
Other comprehensive income	48,602	46,770	84,480	100,211
Balance, ending	52,731	(31,984)	52,731	(31,984)
Pension and other benefits, change in Company tax rate
Income Tax Benefit (Expense)
Other comprehensive income			(96)
Net of tax
Other comprehensive income			(96)
Pension and other benefits, net actuarial losses arising during the period
Pre-tax Amount
Other comprehensive income		(813)		(813)
Income Tax Benefit (Expense)
Other comprehensive income		219		219
Net of tax
Other comprehensive income		(594)		(594)
Pension and other benefits, net change in pension and other benefits
Pre-tax Amount
Other comprehensive income		(813)		(813)
Income Tax Benefit (Expense)
Other comprehensive income		219	(96)	219
Net of tax
Balance, beginning	(28,178)	(28,379)	(28,082)	(28,379)
Other comprehensive income		(594)	(96)	(594)
Balance, ending	(28,178)	(28,973)	(28,178)	(28,973)
Investment securities:
Pre-tax Amount
Unrealized net gains arising during the period	66,071	64,841	115,235	135,364
Reclassification of net losses (gains) to net income	211	(21)	126	2,592
Other comprehensive income	66,282	64,820	115,361	137,956
Income Tax Benefit (Expense)
Unrealized net gains arising during the period	(17,624)	(17,462)	(30,751)	(36,453)
Reclassification of net losses (gains) to net income	(56)	6	34	(698)
Other comprehensive income	(17,680)	(17,456)	(30,785)	(37,151)
Net of tax
Balance, beginning	32,307	(50,375)	(3,667)	(103,816)
Unrealized net gains arising during the period	48,447	47,379	84,484	98,911
Reclassification of net losses (gains) to net income	155	(15)	92	1,894
Other comprehensive income	48,602	47,364	84,576	100,805
Balance, ending	$ 80,909	$ (3,011)	$ 80,909	$ (3,011)