Other Comprehensive Income (Loss) - Changes in Components of Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	$ 1,040,988	$ 1,007,909	$ 982,517	$ 972,060
Current period changes in other comprehensive income (loss)	(34,722)	5,347	(19,194)	16,473
Balance at end of year	1,017,330	991,835	1,017,330	991,835
Accumulated Other Comprehensive (Loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(53,207)	(61,319)	(68,735)	(71,897)
Current period changes in other comprehensive income (loss)	(34,722)	5,347	(19,194)	16,473
Balance at end of year	(87,929)	(55,972)	(87,929)	(55,972)
Unrealized Gains on Debt Securities Available for Sale
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	35,225	9,981	9,313	(13,226)
Current period changes in other comprehensive income (loss)	(2,917)	5,882	22,995	29,637
Balance at end of year	32,308	15,863	32,308	15,863
Unrealized (Losses) on Swaps
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(20,573)	(9,462)	(8,474)	(2,006)
Current period changes in other comprehensive income (loss)	1,785	(1,279)	(10,314)	(8,735)
Balance at end of year	(18,788)	(10,741)	(18,788)	(10,741)
Employee Benefit Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(67,859)	(61,838)	(69,574)	(56,665)
Current period changes in other comprehensive income (loss)	(33,590)	744	(31,875)	(4,429)
Balance at end of year	$ (101,449)	$ (61,094)	(101,449)	(61,094)
Cumulative Effect, Period of Adoption, Adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period				0
Cumulative Effect, Period of Adoption, Adjustment | Accumulated Other Comprehensive (Loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period				(548)
Cumulative Effect, Period of Adoption, Adjustment | Unrealized Gains on Debt Securities Available for Sale
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period				(548)
Cumulative Effect, Period of Adoption, Adjusted Balance
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period				972,060
Cumulative Effect, Period of Adoption, Adjusted Balance | Accumulated Other Comprehensive (Loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period			(68,735)	(72,445)
Cumulative Effect, Period of Adoption, Adjusted Balance | Unrealized Gains on Debt Securities Available for Sale
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period			9,313	(13,774)
Cumulative Effect, Period of Adoption, Adjusted Balance | Unrealized (Losses) on Swaps
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period			(8,474)	(2,006)
Cumulative Effect, Period of Adoption, Adjusted Balance | Employee Benefit Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period			$ (69,574)	$ (56,665)