Securities - Gross Unrealized Losses and Fair Value by Investment Category (Details) $ in Thousands	Mar. 31, 2018 USD ($) security	Dec. 31, 2017 USD ($) security
Available for Sale:
Number of positions, Less than 12 Months | security	207	91
Fair Value, Less than 12 Months	$ 528,747	$ 190,832
Unrealized Losses, Less than 12 Months	$ (8,009)	$ (2,234)
Number of positions, 12 Months or More | security	94	94
Fair value, 12 Months or More	$ 174,424	$ 183,115
Unrealized Losses, 12 Months or More	$ (10,478)	$ (8,254)
Number of positions | security	301	185
Fair value	$ 703,171	$ 373,947
Unrealized Losses	$ (18,487)	$ (10,488)
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	$ 10,045	$ 9,388
Unrealized Losses, 12 Months or More	$ (2,384)	$ (3,054)
Number of positions | security	2	2
Fair value	$ 10,045	$ 9,388
Unrealized Losses	$ (2,384)	$ (3,054)
Other debt securities
Available for Sale:
Number of positions, Less than 12 Months | security	7	2
Fair Value, Less than 12 Months	$ 6,140	$ 756
Unrealized Losses, Less than 12 Months	$ (25)	$ (12)
Number of positions, 12 Months or More | security	2	2
Fair value, 12 Months or More	$ 6,187	$ 6,308
Unrealized Losses, 12 Months or More	$ (181)	$ (64)
Number of positions | security	9	4
Fair value	$ 12,327	$ 7,064
Unrealized Losses	$ (206)	$ (76)
Obligations of other U.S. Government agencies and corporations
Available for Sale:
Number of positions, Less than 12 Months | security	1	1
Fair Value, Less than 12 Months	$ 494	$ 497
Unrealized Losses, Less than 12 Months	$ (6)	$ (3)
Number of positions, 12 Months or More | security	2	2
Fair value, 12 Months or More	$ 1,981	$ 1,999
Unrealized Losses, 12 Months or More	$ (42)	$ (27)
Number of positions | security	3	3
Fair value	$ 2,475	$ 2,496
Unrealized Losses	$ (48)	$ (30)
Obligations of states and political subdivisions
Available for Sale:
Number of positions, Less than 12 Months | security	52	23
Fair Value, Less than 12 Months	$ 31,417	$ 11,860
Unrealized Losses, Less than 12 Months	$ (392)	$ (59)
Number of positions, 12 Months or More | security	12	12
Fair value, 12 Months or More	$ 7,554	$ 7,728
Unrealized Losses, 12 Months or More	$ (372)	$ (210)
Number of positions | security	64	35
Fair value	$ 38,971	$ 19,588
Unrealized Losses	$ (764)	$ (269)
Government agency mortgage backed securities | Residential mortgage-backed securities
Available for Sale:
Number of positions, Less than 12 Months | security	87	29
Fair Value, Less than 12 Months	$ 298,721	$ 64,595
Unrealized Losses, Less than 12 Months	$ (3,942)	$ (659)
Number of positions, 12 Months or More | security	44	44
Fair value, 12 Months or More	$ 84,166	$ 89,414
Unrealized Losses, 12 Months or More	$ (3,986)	$ (2,400)
Number of positions | security	131	73
Fair value	$ 382,887	$ 154,009
Unrealized Losses	$ (7,928)	$ (3,059)
Government agency mortgage backed securities | Commercial mortgage-backed securities
Available for Sale:
Number of positions, Less than 12 Months | security	7	2
Fair Value, Less than 12 Months	$ 8,987	$ 5,629
Unrealized Losses, Less than 12 Months	$ (122)	$ (17)
Number of positions, 12 Months or More | security	3	3
Fair value, 12 Months or More	$ 5,732	$ 5,872
Unrealized Losses, 12 Months or More	$ (318)	$ (217)
Number of positions | security	10	5
Fair value	$ 14,719	$ 11,501
Unrealized Losses	$ (440)	$ (234)
Government agency collateralized mortgage obligations | Residential mortgage-backed securities
Available for Sale:
Number of positions, Less than 12 Months | security	51	33
Fair Value, Less than 12 Months	$ 173,116	$ 102,509
Unrealized Losses, Less than 12 Months	$ (3,394)	$ (1,470)
Number of positions, 12 Months or More | security	29	29
Fair value, 12 Months or More	$ 58,759	$ 62,406
Unrealized Losses, 12 Months or More	$ (3,195)	$ (2,282)
Number of positions | security	80	62
Fair value	$ 231,875	$ 164,915
Unrealized Losses	$ (6,589)	$ (3,752)
Government agency collateralized mortgage obligations | Commercial mortgage-backed securities
Available for Sale:
Number of positions, Less than 12 Months | security	2	1
Fair Value, Less than 12 Months	$ 9,872	$ 4,986
Unrealized Losses, Less than 12 Months	$ (128)	$ (14)
Number of positions, 12 Months or More | security	0	0
Fair value, 12 Months or More	$ 0	$ 0
Unrealized Losses, 12 Months or More	$ 0	$ 0
Number of positions | security	2	1
Fair value	$ 9,872	$ 4,986
Unrealized Losses	$ (128)	$ (14)