Securities - Gross Unrealized Losses and Fair Value by Investment Category (Details) $ in Thousands	Mar. 31, 2019 USD ($) security	Dec. 31, 2018 USD ($) security
Available for Sale:
Number of positions, Less than 12 Months | security	22	168
Fair Value, Less than 12 Months	$ 51,485	$ 509,808
Unrealized Losses, Less than 12 Months	$ (148)	$ (5,163)
Number of positions, 12 Months or More | security	182	171
Fair value, 12 Months or More	$ 438,740	$ 312,698
Unrealized Losses, 12 Months or More	$ (8,795)	$ (13,106)
Number of positions | security	204	339
Fair value	$ 490,225	$ 822,506
Unrealized Losses	$ (8,943)	$ (18,269)
Obligations of other U.S. Government agencies and corporations
Available for Sale:
Number of positions, Less than 12 Months | security	0	0
Fair Value, Less than 12 Months	$ 0	$ 0
Unrealized Losses, Less than 12 Months	$ 0	$ 0
Number of positions, 12 Months or More | security	2	2
Fair value, 12 Months or More	$ 1,494	$ 1,480
Unrealized Losses, 12 Months or More	$ (23)	$ (38)
Number of positions | security	2	2
Fair value	$ 1,494	$ 1,480
Unrealized Losses	$ (23)	$ (38)
Obligations of states and political subdivisions
Available for Sale:
Number of positions, Less than 12 Months | security	1	34
Fair Value, Less than 12 Months	$ 855	$ 22,159
Unrealized Losses, Less than 12 Months	$ (1)	$ (193)
Number of positions, 12 Months or More | security	10	26
Fair value, 12 Months or More	$ 7,309	$ 16,775
Unrealized Losses, 12 Months or More	$ (59)	$ (374)
Number of positions | security	11	60
Fair value	$ 8,164	$ 38,934
Unrealized Losses	$ (60)	$ (567)
Trust preferred securities
Available for Sale:
Number of positions, Less than 12 Months | security	0	0
Fair Value, Less than 12 Months	$ 0	$ 0
Unrealized Losses, Less than 12 Months	$ 0	$ 0
Number of positions, 12 Months or More | security	2	2
Fair value, 12 Months or More	$ 10,246	$ 10,633
Unrealized Losses, 12 Months or More	$ (2,013)	$ (1,726)
Number of positions | security	2	2
Fair value	$ 10,246	$ 10,633
Unrealized Losses	$ (2,013)	$ (1,726)
Other debt securities
Available for Sale:
Number of positions, Less than 12 Months | security	8	12
Fair Value, Less than 12 Months	$ 5,763	$ 19,011
Unrealized Losses, Less than 12 Months	$ (34)	$ (88)
Number of positions, 12 Months or More | security	3	3
Fair value, 12 Months or More	$ 5,752	$ 5,621
Unrealized Losses, 12 Months or More	$ (88)	$ (223)
Number of positions | security	11	15
Fair value	$ 11,515	$ 24,632
Unrealized Losses	$ (122)	$ (311)
Government agency mortgage backed securities | Residential mortgage backed securities
Available for Sale:
Number of positions, Less than 12 Months | security	10	91
Fair Value, Less than 12 Months	$ 28,824	$ 354,731
Unrealized Losses, Less than 12 Months	$ (72)	$ (3,945)
Number of positions, 12 Months or More | security	96	73
Fair value, 12 Months or More	$ 242,222	$ 125,757
Unrealized Losses, 12 Months or More	$ (3,701)	$ (5,181)
Number of positions | security	106	164
Fair value	$ 271,046	$ 480,488
Unrealized Losses	$ (3,773)	$ (9,126)
Government agency mortgage backed securities | Commercial mortgage-backed securities
Available for Sale:
Number of positions, Less than 12 Months | security	0	5
Fair Value, Less than 12 Months	$ 0	$ 6,506
Unrealized Losses, Less than 12 Months	$ 0	$ (74)
Number of positions, 12 Months or More | security	4	4
Fair value, 12 Months or More	$ 7,543	$ 7,468
Unrealized Losses, 12 Months or More	$ (180)	$ (310)
Number of positions | security	4	9
Fair value	$ 7,543	$ 13,974
Unrealized Losses	$ (180)	$ (384)
Government agency collateralized mortgage obligations | Residential mortgage backed securities
Available for Sale:
Number of positions, Less than 12 Months | security	3	24
Fair Value, Less than 12 Months	$ 16,043	$ 97,451
Unrealized Losses, Less than 12 Months	$ (41)	$ (840)
Number of positions, 12 Months or More | security	64	60
Fair value, 12 Months or More	$ 159,212	$ 140,076
Unrealized Losses, 12 Months or More	$ (2,693)	$ (5,142)
Number of positions | security	67	84
Fair value	$ 175,255	$ 237,527
Unrealized Losses	$ (2,734)	$ (5,982)
Government agency collateralized mortgage obligations | Commercial mortgage-backed securities
Available for Sale:
Number of positions, Less than 12 Months | security	0	2
Fair Value, Less than 12 Months	$ 0	$ 9,950
Unrealized Losses, Less than 12 Months	$ 0	$ (23)
Number of positions, 12 Months or More | security	1	1
Fair value, 12 Months or More	$ 4,962	$ 4,888
Unrealized Losses, 12 Months or More	$ (38)	$ (112)
Number of positions | security	1	3
Fair value	$ 4,962	$ 14,838
Unrealized Losses	$ (38)	$ (135)