Other Comprehensive Income (Loss) - Changes in components of other comprehensive income (loss) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of year			$ 472,070	$ 434,852
Current period changes in other comprehensive (loss) income	$ (376)	$ 1,772	(10,326)	2,024
Balance at end of year	941,297	456,443	941,297	456,443
Accumulated Other Comprehensive Loss
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of year	(75,360)	(65,910)	(65,410)	(66,162)
Balance at end of year	(75,736)	(64,138)	(75,736)	(64,138)
Total other comprehensive (loss) income	(75,736)		(75,736)
Unrealized (Losses) Gains on Securities Available-for- Sale
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of year	(17,223)	(8,885)	(7,279)	(9,279)
Current period changes in other comprehensive (loss) income	(2,863)	1,739	(12,743)	2,134
Balance at end of year	(20,086)	(7,146)	(20,086)	(7,146)
Total other comprehensive (loss) income	(20,022)	(7,145)	(20,022)	(7,145)
Swaps
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of year	566	6	224	0
Current period changes in other comprehensive (loss) income	164	(37)	506	(31)
Balance at end of year	730	(31)	730	(31)
Total other comprehensive (loss) income	730	(31)	730	(31)
Employee Benefit Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of year	(58,703)	(57,031)	(58,355)	(56,883)
Current period changes in other comprehensive (loss) income	2,323	70	1,911	(79)
Balance at end of year	(56,380)	(56,961)	(56,380)	(56,961)
Total other comprehensive (loss) income	$ (56,444)	$ (56,962)	$ (56,444)	$ (56,962)