Other Comprehensive Income (Loss) - Changes in components of other comprehensive income (loss) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Jan. 01, 2019	Jan. 01, 2018
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	$ 972,060	$ 472,070
Current period changes in other comprehensive income (loss)	6,625	(9,949)
Cumulative effect of new accounting principle in period of adoption			$ (548)	$ 0
Balance at end of year	994,502	473,905
Accumulated Other Comprehensive Loss
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(71,897)	(65,410)
Current period changes in other comprehensive income (loss)	6,625	(9,949)
Cumulative effect of new accounting principle in period of adoption			(548)
Balance at end of year	(65,820)	(75,359)
Unrealized (Losses) Gains on Securities Available for Sale
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(13,226)	(7,344)
Current period changes in other comprehensive income (loss)	9,405	(9,878)
Cumulative effect of new accounting principle in period of adoption			(548)	0
Balance at end of year	(4,369)	(17,222)
Swaps
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(2,006)	224
Current period changes in other comprehensive income (loss)	(2,780)	342
Cumulative effect of new accounting principle in period of adoption			0	0
Balance at end of year	(4,786)	566
Employee Benefit Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(56,665)	(58,290)
Current period changes in other comprehensive income (loss)	0	(413)
Cumulative effect of new accounting principle in period of adoption			$ 0	$ 0
Balance at end of year	$ (56,665)	$ (58,703)