Securities - Gross Unrealized Losses and Fair Value by Investment Category (Details) $ in Thousands	Jun. 30, 2018 USD ($) security	Dec. 31, 2017 USD ($) security
Available for Sale:
Number of positions, Less than 12 Months | security	231	91
Fair Value, Less than 12 Months	$ 622,240	$ 190,832
Unrealized Losses, Less than 12 Months	$ (10,107)	$ (2,234)
Number of positions, 12 Months or More | security	103	94
Fair value, 12 Months or More	$ 194,840	$ 183,115
Unrealized Losses, 12 Months or More	$ (11,688)	$ (8,254)
Number of positions | security	334	185
Fair value	$ 817,080	$ 373,947
Unrealized Losses	$ (21,795)	$ (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	$ 10,401	$ 9,388
Unrealized Losses, 12 Months or More	$ (2,001)	$ (3,054)
Number of positions | security	2	2
Fair value	$ 10,401	$ 9,388
Unrealized Losses	$ (2,001)	$ (3,054)
Other debt securities
Available for Sale:
Number of positions, Less than 12 Months | security	10	2
Fair Value, Less than 12 Months	$ 10,011	$ 756
Unrealized Losses, Less than 12 Months	$ (110)	$ (12)
Number of positions, 12 Months or More | security	2	2
Fair value, 12 Months or More	$ 5,815	$ 6,308
Unrealized Losses, 12 Months or More	$ (192)	$ (64)
Number of positions | security	12	4
Fair value	$ 15,826	$ 7,064
Unrealized Losses	$ (302)	$ (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	$ 492	$ 497
Unrealized Losses, Less than 12 Months	$ (8)	$ (3)
Number of positions, 12 Months or More | security	2	2
Fair value, 12 Months or More	$ 1,980	$ 1,999
Unrealized Losses, 12 Months or More	$ (41)	$ (27)
Number of positions | security	3	3
Fair value	$ 2,472	$ 2,496
Unrealized Losses	$ (49)	$ (30)
Obligations of states and political subdivisions
Available for Sale:
Number of positions, Less than 12 Months | security	51	23
Fair Value, Less than 12 Months	$ 32,251	$ 11,860
Unrealized Losses, Less than 12 Months	$ (386)	$ (59)
Number of positions, 12 Months or More | security	13	12
Fair value, 12 Months or More	$ 7,800	$ 7,728
Unrealized Losses, 12 Months or More	$ (372)	$ (210)
Number of positions | security	64	35
Fair value	$ 40,051	$ 19,588
Unrealized Losses	$ (758)	$ (269)
Government agency mortgage backed securities | Residential mortgage backed securities:
Available for Sale:
Number of positions, Less than 12 Months | security	105	29
Fair Value, Less than 12 Months	$ 361,859	$ 64,595
Unrealized Losses, Less than 12 Months	$ (5,623)	$ (659)
Number of positions, 12 Months or More | security	47	44
Fair value, 12 Months or More	$ 88,914	$ 89,414
Unrealized Losses, 12 Months or More	$ (4,589)	$ (2,400)
Number of positions | security	152	73
Fair value	$ 450,773	$ 154,009
Unrealized Losses	$ (10,212)	$ (3,059)
Government agency mortgage backed securities | Commercial mortgage-backed securities
Available for Sale:
Number of positions, Less than 12 Months | security	8	2
Fair Value, Less than 12 Months	$ 14,530	$ 5,629
Unrealized Losses, Less than 12 Months	$ (178)	$ (17)
Number of positions, 12 Months or More | security	3	3
Fair value, 12 Months or More	$ 5,659	$ 5,872
Unrealized Losses, 12 Months or More	$ (352)	$ (217)
Number of positions | security	11	5
Fair value	$ 20,189	$ 11,501
Unrealized Losses	$ (530)	$ (234)
Government agency collateralized mortgage obligations | Residential mortgage backed securities:
Available for Sale:
Number of positions, Less than 12 Months | security	52	33
Fair Value, Less than 12 Months	$ 178,776	$ 102,509
Unrealized Losses, Less than 12 Months	$ (3,538)	$ (1,470)
Number of positions, 12 Months or More | security	34	29
Fair value, 12 Months or More	$ 74,271	$ 62,406
Unrealized Losses, 12 Months or More	$ (4,141)	$ (2,282)
Number of positions | security	86	62
Fair value	$ 253,047	$ 164,915
Unrealized Losses	$ (7,679)	$ (3,752)
Government agency collateralized mortgage obligations | Commercial mortgage-backed securities
Available for Sale:
Number of positions, Less than 12 Months | security	4	1
Fair Value, Less than 12 Months	$ 24,321	$ 4,986
Unrealized Losses, Less than 12 Months	$ (264)	$ (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	4	1
Fair value	$ 24,321	$ 4,986
Unrealized Losses	$ (264)	$ (14)