Investment Securities - Number, fair value and gross unrealized holding losses (Details) $ in Thousands	Dec. 31, 2018 USD ($) investment_security	Dec. 31, 2017 USD ($) investment_security
Number
Less than 12 months | investment_security	105	184
12 months or Longer | investment_security	244	76
Total | investment_security	349	260
Fair Value
Less than 12 months	$ 190,770	$ 320,068
12 months or Longer	442,464	140,739
Total	633,234	460,807
Gross Unrealized Holding Losses
Less than 12 months	(1,609)	(2,159)
12 months or Longer	(12,600)	(3,356)
Total	$ (14,209)	$ (5,515)
Number
Less than 12 months | investment_security	3	2
12 months or Longer | investment_security	3	1
Total | investment_security	6	3
Fair Value
Less than 12 months	$ 11,256	$ 10,745
12 months or Longer	15,741	6,198
Total	26,997	16,943
Gross Unrealized Holding Losses
Less than 12 months	(81)	(133)
12 months or Longer	(605)	(76)
Total	$ (686)	$ (209)
Total investment securities
Number, Less than 12 months (in investments) | investment_security	108	186
Number, 12 months or Longer (in investments) | investment_security	247	77
Number, Total (in investments) | investment_security	355	263
Fair Value, Less than 12 months	$ 202,026	$ 330,813
Fair Value, 12 months or Longer	458,205	146,937
Fair Value, Total	660,231	477,750
Gross Unrealized Holding Losses, Less than 12 months	(1,690)	(2,292)
Gross Unrealized Holding Losses, 12 months or longer	(13,205)	(3,432)
Gross Unrealized Holding Losses, Total	$ (14,895)	$ (5,724)
Agency
Number
Less than 12 months | investment_security	15	6
12 months or Longer | investment_security	6	0
Total | investment_security	21	6
Fair Value
Less than 12 months	$ 26,229	$ 13,754
12 months or Longer	10,434	0
Total	36,663	13,754
Gross Unrealized Holding Losses
Less than 12 months	(333)	(78)
12 months or Longer	(186)	0
Total	$ (519)	$ (78)
Corporate debt
Number
Less than 12 months | investment_security	9	4
12 months or Longer | investment_security	8	2
Total | investment_security	17	6
Fair Value
Less than 12 months	$ 47,805	$ 10,079
12 months or Longer	19,369	6,076
Total	67,174	16,155
Gross Unrealized Holding Losses
Less than 12 months	(471)	(64)
12 months or Longer	(435)	(130)
Total	$ (906)	$ (194)
Municipal bonds
Number
Less than 12 months | investment_security	60	103
12 months or Longer | investment_security	102	30
Total | investment_security	162	133
Fair Value
Less than 12 months	$ 45,083	$ 61,313
12 months or Longer	69,693	15,658
Total	114,776	76,971
Gross Unrealized Holding Losses
Less than 12 months	(369)	(268)
12 months or Longer	(1,856)	(475)
Total	$ (2,225)	$ (743)
Collateralized mortgage obligation: residential
Number
Less than 12 months | investment_security	1	5
12 months or Longer | investment_security	8	3
Total | investment_security	9	8
Fair Value
Less than 12 months	$ 814	$ 13,971
12 months or Longer	18,104	8,943
Total	18,918	22,914
Gross Unrealized Holding Losses
Less than 12 months	(1)	(149)
12 months or Longer	(424)	(186)
Total	$ (425)	$ (335)
Mortgage-backed securities: residential
Number
Less than 12 months | investment_security	20	66
12 months or Longer | investment_security	120	41
Total | investment_security	140	107
Fair Value
Less than 12 months	$ 70,839	$ 220,951
12 months or Longer	324,864	110,062
Total	395,703	331,013
Gross Unrealized Holding Losses
Less than 12 months	(435)	(1,600)
12 months or Longer	(9,699)	(2,565)
Total	$ (10,134)	$ (4,165)
Number
Less than 12 months | investment_security	3	2
12 months or Longer | investment_security	3	1
Total | investment_security	6	3
Fair Value
Less than 12 months	$ 11,256	$ 10,745
12 months or Longer	15,741	6,198
Total	26,997	16,943
Gross Unrealized Holding Losses
Less than 12 months	(81)	(133)
12 months or Longer	(605)	(76)
Total	$ (686)	$ (209)
Other
Number
Less than 12 months | investment_security	0	0
12 months or Longer | investment_security	0	0
Total | investment_security	0	0
Fair Value
Less than 12 months	$ 0	$ 0
12 months or Longer	0	0
Total	0	0
Gross Unrealized Holding Losses
Less than 12 months	0	0
12 months or Longer	0	0
Total	$ 0	$ 0