Securities - Schedule of Unrealized Loss and Fair Value on Investments (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Fair Value
Less than 12 months	$ 843,091	$ 326,177
12 months or longer	434,034	440,333
Total	1,277,125	766,510
Unrealized Losses
Less than 12 months	(17,268)	(2,134)
12 months or longer	(22,853)	(10,540)
Total	(40,121)	(12,674)
Collateralized loan obligations
Fair Value
Less than 12 months	255,030	14,933
12 months or longer	0	0
Total	255,030	14,933
Unrealized Losses
Less than 12 months	(585)	(68)
12 months or longer	0	0
Total	(585)	(68)
Mortgage-backed securities and collateralized mortgage obligations of U.S. Government Sponsored Entities
Fair Value
Less than 12 months	456,356	304,723
12 months or longer	416,936	413,725
Total	873,292	718,448
Unrealized Losses
Less than 12 months	(15,703)	(2,047)
12 months or longer	(22,514)	(10,146)
Total	(38,217)	(12,193)
Private collateralized mortgage obligations
Fair Value
Less than 12 months	84,026	0
12 months or longer	13,836	20,744
Total	97,862	20,744
Unrealized Losses
Less than 12 months	(251)	0
12 months or longer	(197)	(338)
Total	(448)	(338)
Obligations of state and political subdivisions
Fair Value
Less than 12 months	40,136	5,414
12 months or longer	3,262	5,864
Total	43,398	11,278
Unrealized Losses
Less than 12 months	(672)	(14)
12 months or longer	(142)	(56)
Total	(814)	(70)
U.S. Treasury securities and obligations of U.S. Government Entities
Fair Value
Less than 12 months	7,543	1,107
12 months or longer	0	0
Total	7,543	1,107
Unrealized Losses
Less than 12 months	(57)	(5)
12 months or longer	0	0
Total	$ (57)	$ (5)