Other Comprehensive Income (Loss) - Changes in components of other comprehensive income (loss) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of quarter			$ 472,070	$ 434,852
Current period changes in other comprehensive (loss) income	$ (6,034)	$ 17,958	(16,360)	19,982
Balance at end of year	947,046	475,914	947,046	475,914
Accumulated Other Comprehensive Loss
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of quarter	(75,736)	(64,138)	(65,410)	(66,162)
Balance at end of year	(81,770)	(46,180)	(81,770)	(46,180)
Unrealized (Losses) Gains on Securities Available-for- Sale
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of quarter	(20,086)	(7,146)	(7,344)	(9,279)
Current period changes in other comprehensive (loss) income	(5,366)	3,008	(18,108)	5,141
Balance at end of year	(25,452)	(4,138)	(25,452)	(4,138)
Swaps
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of quarter	730	(31)	224	0
Current period changes in other comprehensive (loss) income	1,240	93	1,746	62
Balance at end of year	1,970	62	1,970	62
Employee Benefit Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of quarter	(56,380)	(56,961)	(58,290)	(56,883)
Current period changes in other comprehensive (loss) income	(1,908)	14,857	2	14,779
Balance at end of year	$ (58,288)	$ (42,104)	$ (58,288)	$ (42,104)