Shareholders' Equity - Changes in Balances of Component of Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	$ 1,741,117	$ 1,660,702
Other comprehensive income (loss) before reclassification	(15,817)	31,026
Amounts reclassified from accumulated other comprehensive income (loss)	362	272
Other comprehensive income (loss), net of tax amount	(15,455)	31,298
Balance	1,759,705	1,652,399
Securities Available for Sale and Transferred Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	17,331	(8,017)
Other comprehensive income (loss) before reclassification	(15,817)	31,026
Other comprehensive income (loss), net of tax amount	(15,817)	31,026
Balance	1,514	23,009
Defined Benefit Pension Items [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	(18,382)	(15,583)
Amounts reclassified from accumulated other comprehensive income (loss)	362	272
Other comprehensive income (loss), net of tax amount	362	272
Balance	(18,020)	(15,311)
Accumulated Other Comprehensive Income (Loss) [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	(1,051)	(23,600)
Balance	$ (16,506)	$ 7,698