Accumulated Other Comprehensive Income (Loss) - Changes (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Pre-tax Amount
Other comprehensive income (loss), Pre-tax Amount	$ 18,729	$ 66,282	$ (83,607)	$ 115,361
Income Tax Benefit (Expense)
Other comprehensive income (loss), Income Tax Benefit (Expense)	(4,996)	(17,680)	22,301	(30,881)
Net of tax
Balance	2,683,630	2,664,685	2,744,104	2,640,258
Other comprehensive income (loss)	13,733	48,602	(61,306)	84,480
Balance	2,731,341	2,701,897	2,731,341	2,701,897
Accumulated other comprehensive Income (Loss)
Pre-tax Amount
Accumulated other comprehensive loss, beginning balance, Pre-tax Amount	(59,238)	5,629	43,098	(43,450)
Accumulated other comprehensive loss, ending balance, Pre-tax Amount	(40,509)	71,911	(40,509)	71,911
Income Tax Benefit (Expense)
Accumulated other comprehensive loss, beginning balance, Income Tax Benefit (Expense)	15,803	(1,500)	(11,494)	11,701
Accumulated other comprehensive loss, ending balance, Income Tax Benefit (Expense)	10,807	(19,180)	10,807	(19,180)
Net of tax
Balance	(43,435)	4,129	31,604	(31,749)
Other comprehensive income (loss)	13,733	48,602	(61,306)	84,480
Balance	(29,702)	52,731	(29,702)	52,731
Pension and other benefits:
Income Tax Benefit (Expense)
Other comprehensive income (loss), Income Tax Benefit (Expense)				(96)
Net of tax
Balance	(31,737)	(28,178)	(31,737)	(28,082)
Other comprehensive income (loss)				(96)
Balance	(31,737)	(28,178)	(31,737)	(28,178)
Pension and other benefits, change in Company tax rate
Income Tax Benefit (Expense)
Unrealized net gains arising during the period, Income Tax Benefit (Expense)				(96)
Net of tax
Unrealized net gains arising during the period, Net of tax				(96)
Investment securities:
Pre-tax Amount
Unrealized net gains (losses) arising during the period, Pre-tax Amount	18,831	66,071	(83,505)	115,235
Reclassification of net losses (gains) to net income, Pre-tax Amount	(102)	211	(102)	126
Other comprehensive income (loss), Pre-tax Amount	18,729	66,282	(83,607)	115,361
Income Tax Benefit (Expense)
Unrealized net gains arising during the period, Income Tax Benefit (Expense)	(5,023)	(17,624)	22,274	(30,751)
Reclassification of net gains to net income, Income Tax Benefit (Expense)	27	(56)	27	(34)
Other comprehensive income (loss), Income Tax Benefit (Expense)	(4,996)	(17,680)	22,301	(30,785)
Net of tax
Balance	(11,698)	32,307	63,341	(3,667)
Unrealized net gains arising during the period, Net of tax	13,808	48,447	(61,231)	84,484
Reclassification of net gains to net income, Net of tax	(75)	155	(75)	92
Other comprehensive income (loss)	13,733	48,602	(61,306)	84,576
Balance	$ 2,035	$ 80,909	$ 2,035	$ 80,909