Securities Securities - Available for sale (Details) - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Available-for-sale Securities [Abstract]
Amortized Cost	$ 2,513,540,000	$ 1,310,071,000
Gross Unrealized Gain	19,244,000	8,557,000
Gross Unrealized Loss	17,501,000	20,082,000
Securities available-for-sale, at fair value	2,515,283,219	1,298,546,056
Available-for-sale, Amortized Cost
Due in one year of less	33,251,000
Due in one year to five years	65,214,000
Due in five to ten years	185,337,000
Due after ten years	785,767,000
Mortgage-backed securities	1,270,625,000
Asset-backed securities	173,346,000
Amortized Cost	2,513,540,000
Available-for-sale, Fair Value
Due in one year or less	33,196,000
Due in one year to five years	65,775,000
Due in five years to ten years	187,198,000
Due after ten years	792,003,000
Mortgage-backed securities	1,263,819,000
Asset-backed securities	173,292,000
Securities available-for-sale, at fair value	2,515,283,219	1,298,546,056
State and Municipal Securities
Available-for-sale Securities [Abstract]
Amortized Cost	774,949,000	211,581,000
Gross Unrealized Gain	12,251,000	4,103,000
Gross Unrealized Loss	2,588,000	2,964,000
Securities available-for-sale, at fair value	784,612,000	212,720,000
Available-for-sale, Fair Value
Securities available-for-sale, at fair value	784,612,000	212,720,000
U.S. Government Agency Securities
Available-for-sale Securities [Abstract]
Amortized Cost	182,500,000	22,306,000
Gross Unrealized Gain	67,000	0
Gross Unrealized Loss	1,766,000	537,000
Securities available-for-sale, at fair value	180,801,000	21,769,000
Available-for-sale, Fair Value
Securities available-for-sale, at fair value	180,801,000	21,769,000
Mortgage-backed Agency Securities
Available-for-sale Securities [Abstract]
Amortized Cost	1,270,625,000	988,008,000
Gross Unrealized Gain	5,318,000	4,304,000
Gross Unrealized Loss	12,124,000	15,686,000
Securities available-for-sale, at fair value	1,263,819,000	976,626,000
Available-for-sale, Fair Value
Securities available-for-sale, at fair value	1,263,819,000	976,626,000
US Treasury Securities
Available-for-sale Securities [Abstract]
Amortized Cost	30,505,000	250,000
Gross Unrealized Gain	0	0
Gross Unrealized Loss	60,000	0
Securities available-for-sale, at fair value	30,445,000	250,000
Available-for-sale, Fair Value
Securities available-for-sale, at fair value	30,445,000	250,000
Asset-backed Securities
Available-for-sale Securities [Abstract]
Amortized Cost	173,346,000	79,318,000
Gross Unrealized Gain	262,000	111,000
Gross Unrealized Loss	316,000	849,000
Securities available-for-sale, at fair value	173,292,000	78,580,000
Available-for-sale, Fair Value
Securities available-for-sale, at fair value	173,292,000	78,580,000
Corporate Notes
Available-for-sale Securities [Abstract]
Amortized Cost	81,615,000	8,608,000
Gross Unrealized Gain	1,346,000	39,000
Gross Unrealized Loss	647,000	46,000
Securities available-for-sale, at fair value	82,314,000	8,601,000
Available-for-sale, Fair Value
Securities available-for-sale, at fair value	$ 82,314,000	$ 8,601,000