Accumulated Other Comprehensive Income (Loss) - Changes, Net of Tax (Details) - USD ($)	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Net of tax
Balance	$ 2,744,104,000	$ 2,640,258,000	$ 2,524,839,000
Net income	265,735,000	185,754,000	284,392,000
Other comprehensive income (loss)	(153,297,000)	63,353,000	100,446,000
Balance	2,656,912,000	2,744,104,000	2,640,258,000
Incremental non-credit-related impairment loss recognized in earnings on securities	0	0
Non-credit OTTI loss on securities			0
Accumulated Other Comprehensive Income (Loss)
Net of tax
Balance	31,604,000	(31,749,000)	(132,195,000)
Other comprehensive income (loss)	(153,297,000)	63,353,000	100,446,000
Balance	(121,693,000)	31,604,000	(31,749,000)
Unfunded Pension Liability
Net of tax
Balance	(31,737,000)	(28,082,000)	(28,379,000)
Other comprehensive income (loss)	7,347,000	(3,655,000)	297,000
Balance	(24,390,000)	(31,737,000)	(28,082,000)
Unrealized Loss on Securities
Net of tax
Balance	63,341,000	(3,667,000)	(103,816,000)
Other comprehensive income (loss)	(160,644,000)	67,008,000	100,149,000
Balance	$ (97,303,000)	63,341,000	(3,667,000)
Reclassification of net losses (gains) to net income, Pre-tax Amount		$ (114,000)	$ (2,715,000)