Other Comprehensive Income (Loss) - Changes in components of other comprehensive income (loss) (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Sep. 30, 2017	Jan. 01, 2019	Jan. 01, 2018	Oct. 01, 2017
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	$ 475,914	$ 972,060	$ 472,070	$ 439,664
Adjusted balance					$ 972,060
Current period changes in other comprehensive income (loss)	(19,230)	3,710	(6,487)	5,178
Current period changes in other comprehensive (loss) income	(7,522)
Balance at end of year	472,070	982,517	972,060	475,914
Accumulated Other Comprehensive (Loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(46,180)	(71,897)	(65,410)	(51,358)
Reclassification of tax effect resulting from adoption					(548)		$ (11,708)
Adjusted balance					(72,445)	$ (65,410)
Current period changes in other comprehensive income (loss)		3,710	(6,487)	5,178
Current period changes in other comprehensive (loss) income	(7,522)
Balance at end of year	(65,410)	(68,735)	(71,897)	(46,180)
Unrealized (Losses) Gains on Debt Securities Available for Sale
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(4,137)	(13,162)	(7,279)	5,664
Reclassification of tax effect resulting from adoption					(548)		(1,314)
Adjusted balance					(13,710)	(7,279)
Current period changes in other comprehensive income (loss)		23,087	(5,883)	(9,801)
Current period changes in other comprehensive (loss) income	(1,828)
Balance at end of year	(7,279)	9,377	(13,162)	(4,137)
Unrealized (Losses) on Swaps
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	62	(2,006)	224	0
Reclassification of tax effect resulting from adoption							40
Adjusted balance					(2,006)	224
Current period changes in other comprehensive income (loss)		(6,468)	(2,230)	62
Current period changes in other comprehensive (loss) income	122
Balance at end of year	224	(8,474)	(2,006)	62
Employee Benefit Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(42,105)	(56,729)	(58,355)	(57,022)
Reclassification of tax effect resulting from adoption							$ (10,434)
Adjusted balance					$ (56,729)	$ (58,355)
Current period changes in other comprehensive income (loss)		(12,909)	1,626	14,917
Current period changes in other comprehensive (loss) income	(5,816)
Balance at end of year	$ (58,355)	$ (69,638)	$ (56,729)	$ (42,105)