Securities - Gross Unrealized Losses and Fair Value by Investment Category (Details) $ in Thousands	Sep. 30, 2018 USD ($) security	Dec. 31, 2017 USD ($) security
Available for Sale:
Number of positions, Less than 12 Months | security	277	91
Fair Value, Less than 12 Months	$ 733,743	$ 190,832
Unrealized Losses, Less than 12 Months	$ (12,538)	$ (2,234)
Number of positions, 12 Months or More | security	113	94
Fair value, 12 Months or More	$ 233,707	$ 183,115
Unrealized Losses, 12 Months or More	$ (14,445)	$ (8,254)
Number of positions | security	390	185
Fair value	$ 967,450	$ 373,947
Unrealized Losses	$ (26,983)	$ (10,488)
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	$ 490	$ 497
Unrealized Losses, Less than 12 Months	$ (10)	$ (3)
Number of positions, 12 Months or More | security	2	2
Fair value, 12 Months or More	$ 1,982	$ 1,999
Unrealized Losses, 12 Months or More	$ (38)	$ (27)
Number of positions | security	3	3
Fair value	$ 2,472	$ 2,496
Unrealized Losses	$ (48)	$ (30)
Obligations of states and political subdivisions
Available for Sale:
Number of positions, Less than 12 Months | security	79	23
Fair Value, Less than 12 Months	$ 52,161	$ 11,860
Unrealized Losses, Less than 12 Months	$ (758)	$ (59)
Number of positions, 12 Months or More | security	12	12
Fair value, 12 Months or More	$ 7,432	$ 7,728
Unrealized Losses, 12 Months or More	$ (435)	$ (210)
Number of positions | security	91	35
Fair value	$ 59,593	$ 19,588
Unrealized Losses	$ (1,193)	$ (269)
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	$ 10,304	$ 9,388
Unrealized Losses, 12 Months or More	$ (2,047)	$ (3,054)
Number of positions | security	2	2
Fair value	$ 10,304	$ 9,388
Unrealized Losses	$ (2,047)	$ (3,054)
Other debt securities
Available for Sale:
Number of positions, Less than 12 Months | security	14	2
Fair Value, Less than 12 Months	$ 13,823	$ 756
Unrealized Losses, Less than 12 Months	$ (208)	$ (12)
Number of positions, 12 Months or More | security	3	2
Fair value, 12 Months or More	$ 6,020	$ 6,308
Unrealized Losses, 12 Months or More	$ (239)	$ (64)
Number of positions | security	17	4
Fair value	$ 19,843	$ 7,064
Unrealized Losses	$ (447)	$ (76)
Government agency mortgage backed securities | Residential mortgage backed securities
Available for Sale:
Number of positions, Less than 12 Months | security	119	29
Fair Value, Less than 12 Months	$ 440,200	$ 64,595
Unrealized Losses, Less than 12 Months	$ (7,536)	$ (659)
Number of positions, 12 Months or More | security	52	44
Fair value, 12 Months or More	$ 94,329	$ 89,414
Unrealized Losses, 12 Months or More	$ (5,353)	$ (2,400)
Number of positions | security	171	73
Fair value	$ 534,529	$ 154,009
Unrealized Losses	$ (12,889)	$ (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	$ 11,396	$ 5,629
Unrealized Losses, Less than 12 Months	$ (188)	$ (17)
Number of positions, 12 Months or More | security	2	3
Fair value, 12 Months or More	$ 5,072	$ 5,872
Unrealized Losses, 12 Months or More	$ (374)	$ (217)
Number of positions | security	10	5
Fair value	$ 16,468	$ 11,501
Unrealized Losses	$ (562)	$ (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	$ 186,677	$ 102,509
Unrealized Losses, Less than 12 Months	$ (3,468)	$ (1,470)
Number of positions, 12 Months or More | security	40	29
Fair value, 12 Months or More	$ 108,568	$ 62,406
Unrealized Losses, 12 Months or More	$ (5,959)	$ (2,282)
Number of positions | security	91	62
Fair value	$ 295,245	$ 164,915
Unrealized Losses	$ (9,427)	$ (3,752)
Government agency collateralized mortgage obligations | Commercial mortgage-backed securities
Available for Sale:
Number of positions, Less than 12 Months | security	5	1
Fair Value, Less than 12 Months	$ 28,996	$ 4,986
Unrealized Losses, Less than 12 Months	$ (370)	$ (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	5	1
Fair value	$ 28,996	$ 4,986
Unrealized Losses	$ (370)	$ (14)