Investment Securities - Number, fair value and gross unrealized holding losses (Details) $ in Thousands	Dec. 31, 2017 USD ($) investment_security	Dec. 31, 2016 USD ($) investment_security
Number
Less than 12 months | investment_security	184	215
12 months or Longer | investment_security	76	4
Total | investment_security	260	219
Fair Value
Less than 12 months	$ 320,068	$ 262,191
12 months or Longer	140,739	16,039
Total	460,807	278,230
Gross Unrealized Holding Losses
Less than 12 months	(2,159)	(4,984)
12 months or Longer	(3,356)	(519)
Total	$ (5,515)	$ (5,503)
Number
Less than 12 months | investment_security	2	1
12 months or Longer | investment_security	1	0
Total | investment_security	3	1
Fair Value
Less than 12 months	$ 10,745	$ 7,271
12 months or Longer	6,198	0
Total	16,943	7,271
Gross Unrealized Holding Losses
Less than 12 months	(133)	(104)
12 months or Longer	(76)	0
Total	$ (209)	$ (104)
Total investment securities
Number, Less than 12 months (in investments) | investment_security	186	216
Number, 12 months or Longer (in investments) | investment_security	77	4
Number, Total (in investments) | investment_security	263	220
Fair Value, Less than 12 months	$ 330,813	$ 269,462
Fair Value, 12 months or Longer	146,937	16,039
Fair Value, Total	477,750	285,501
Gross Unrealized Holding Losses, Less than 12 months	(2,292)	(5,088)
Gross Unrealized Holding Losses, 12 months or longer	(3,432)	(519)
Gross Unrealized Holding Losses, Total	$ (5,724)	$ (5,607)
Agency
Number
Less than 12 months | investment_security	6
12 months or Longer | investment_security	0
Total | investment_security	6
Fair Value
Less than 12 months	$ 13,754
12 months or Longer	0
Total	13,754
Gross Unrealized Holding Losses
Less than 12 months	(78)
12 months or Longer	0
Total	$ (78)
Corporate
Number
Less than 12 months | investment_security	4	3
12 months or Longer | investment_security	2	0
Total | investment_security	6	3
Fair Value
Less than 12 months	$ 10,079	$ 7,609
12 months or Longer	6,076	0
Total	16,155	7,609
Gross Unrealized Holding Losses
Less than 12 months	(64)	(205)
12 months or Longer	(130)	0
Total	$ (194)	$ (205)
Municipal bonds
Number
Less than 12 months | investment_security	103	152
12 months or Longer | investment_security	30	0
Total | investment_security	133	152
Fair Value
Less than 12 months	$ 61,313	$ 85,750
12 months or Longer	15,658	0
Total	76,971	85,750
Gross Unrealized Holding Losses
Less than 12 months	(268)	(1,690)
12 months or Longer	(475)	0
Total	$ (743)	$ (1,690)
Collateralized mortgage obligation: residential
Number
Less than 12 months | investment_security	5	5
12 months or Longer | investment_security	3	0
Total | investment_security	8	5
Fair Value
Less than 12 months	$ 13,971	$ 19,092
12 months or Longer	8,943	0
Total	22,914	19,092
Gross Unrealized Holding Losses
Less than 12 months	(149)	(173)
12 months or Longer	(186)	0
Total	$ (335)	$ (173)
Mortgage-backed securities: residential
Number
Less than 12 months | investment_security	66	55
12 months or Longer | investment_security	41	4
Total | investment_security	107	59
Fair Value
Less than 12 months	$ 220,951	$ 149,740
12 months or Longer	110,062	16,039
Total	331,013	165,779
Gross Unrealized Holding Losses
Less than 12 months	(1,600)	(2,916)
12 months or Longer	(2,565)	(519)
Total	$ (4,165)	$ (3,435)
Number
Less than 12 months | investment_security	2	1
12 months or Longer | investment_security	1	0
Total | investment_security	3	1
Fair Value
Less than 12 months	$ 10,745	$ 7,271
12 months or Longer	6,198	0
Total	16,943	7,271
Gross Unrealized Holding Losses
Less than 12 months	(133)	(104)
12 months or Longer	(76)	0
Total	$ (209)	$ (104)