Accumulated Other Comprehensive Income (Loss) - Changes (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Pre-tax Amount
Other comprehensive income (loss), Pre-tax Amount	$ (43,316)	$ (2,015)	$ (126,923)	$ 113,346
Income Tax Benefit (Expense)
Other comprehensive income (loss), Income Tax Benefit (Expense)	11,555	538	33,856	(30,343)
Net of tax
Balance	2,731,341	2,701,897	2,744,104	2,640,258
Other comprehensive (loss) income	(31,761)	(1,477)	(93,067)	83,003
Balance	2,711,734	2,733,934	2,711,734	2,733,934
Accumulated other comprehensive Income (Loss)
Pre-tax Amount
Accumulated other comprehensive income (loss), beginning balance, Pre-tax Amount	(40,509)	71,911	43,098	(43,450)
Accumulated other comprehensive (loss) income, ending balance, Pre-tax Amount	(83,825)	69,896	(83,825)	69,896
Income Tax Benefit (Expense)
Accumulated other comprehensive income (loss), beginning balance, Income Tax Benefit (Expense)	10,807	(19,180)	(11,494)	11,701
Accumulated other comprehensive loss, ending balance, Income Tax Benefit (Expense)	22,362	(18,642)	22,362	(18,642)
Net of tax
Balance	(29,702)	52,731	31,604	(31,749)
Other comprehensive (loss) income	(31,761)	(1,477)	(93,067)	83,003
Balance	(61,463)	51,254	(61,463)	51,254
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 (loss) income				(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 losses (gains) arising during the period, Income Tax Benefit (Expense)				(96)
Net of tax
Unrealized net (losses) 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	(43,316)	(1,991)	(126,821)	113,244
Reclassification of net losses (gains) to net income, Pre-tax Amount		(24)	(102)	102
Other comprehensive income (loss), Pre-tax Amount	(43,316)	(2,015)	(126,923)	113,346
Income Tax Benefit (Expense)
Unrealized net losses (gains) arising during the period, Income Tax Benefit (Expense)	11,555	532	33,829	(30,220)
Reclassification of net gains (losses) to net income, Income Tax Benefit (Expense)		(6)	(27)	27
Other comprehensive income (loss), Income Tax Benefit (Expense)	11,555	538	33,856	(30,247)
Net of tax
Balance	2,035	80,909	63,341	(3,667)
Unrealized net (losses) gains arising during the period, Net of tax	(31,761)	(1,459)	(92,992)	83,024
Reclassification of net (gains) losses to net income, Net of tax		(18)	(75)	75
Other comprehensive (loss) income	(31,761)	(1,477)	(93,067)	83,099
Balance	$ (29,726)	$ 79,432	$ (29,726)	$ 79,432