Securities - Amortized Cost and Fair Value of Securities (Details) - USD ($)	3 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2018	Mar. 31, 2019
Securities available-for-sale [Abstract]
Amortized Cost		$ 3,131,227,000	$ 3,238,395,000
Gross Unrealized Gains		7,934,000	35,522,000
Gross Unrealized Losses		55,475,000	23,911,000
Securities available-for-sale, at fair value		3,083,686,000	3,250,006,000
Available-for-sale, Amortized Cost [Abstract]
Due in one year or less			63,866,000
Due in one year to five years			22,546,000
Due in five years to ten years			103,720,000
Due after ten years			1,406,074,000
Mortgage-backed securities			1,372,248,000
Asset-backed securities			269,941,000
Amortized Cost			3,238,395,000
Available-for-sale, Fair Value [Abstract]
Due in one year or less			63,860,000
Due in one year to five years			22,533,000
Due in five years to ten years			103,272,000
Due after ten years			1,431,610,000
Mortgage-backed securities			1,359,721,000
Asset-backed securities			269,010,000
Securities available-for-sale, at fair value		3,083,686,000	3,250,006,000
Schedule of Held-to-maturity Securities [Line Items]
Available-for-sale securities transferred to Held-to-Maturity		179,800,000
Net unrealized after tax loss	$ 2,200,000
Securities held-to-maturity [Abstract]
Amortized Cost		194,282,000	194,043,000
Gross Unrealized Gains		152,000	4,979,000
Gross Unrealized Losses		1,303,000	12,000
Securities held-to-maturity, fair value		193,131,000	199,010,000
Held-to-maturity, Amortized Cost [Abstract]
Due in one year or less			325,000
Due in one year to five years			5,679,000
Due in five years to ten years			7,962,000
Due after ten years			180,077,000
Mortgage-backed securities			0
Asset-backed securities			0
Amortized Cost		194,282,000	194,043,000
Held-to-maturity, Fair Value [Abstract]
Due in one year or less			326,000
Due in one year to five years			5,678,000
Due in five years to ten years			8,003,000
Due after ten years			185,003,000
Mortgage-backed securities			0
Asset-backed securities			0
Fair Value		193,131,000	199,010,000
U.S. Treasury securities
Securities available-for-sale [Abstract]
Amortized Cost		30,325,000	48,612,000
Gross Unrealized Gains		0	5,000
Gross Unrealized Losses		25,000	5,000
Securities available-for-sale, at fair value		30,300,000	48,612,000
Available-for-sale, Fair Value [Abstract]
Securities available-for-sale, at fair value		30,300,000	48,612,000
U.S. government agency securities
Securities available-for-sale [Abstract]
Amortized Cost		71,456,000	68,219,000
Gross Unrealized Gains		49,000	25,000
Gross Unrealized Losses		1,346,000	1,275,000
Securities available-for-sale, at fair value		70,159,000	66,969,000
Available-for-sale, Fair Value [Abstract]
Securities available-for-sale, at fair value		70,159,000	66,969,000
Mortgage-backed securities
Securities available-for-sale [Abstract]
Amortized Cost		1,336,469,000	1,372,248,000
Gross Unrealized Gains		3,110,000	3,798,000
Gross Unrealized Losses		28,634,000	16,325,000
Securities available-for-sale, at fair value		1,310,945,000	1,359,721,000
Available-for-sale, Fair Value [Abstract]
Securities available-for-sale, at fair value		1,310,945,000	1,359,721,000
State and municipal securities
Securities available-for-sale [Abstract]
Amortized Cost		1,244,471,000	1,423,313,000
Gross Unrealized Gains		3,785,000	30,366,000
Gross Unrealized Losses		18,602,000	2,948,000
Securities available-for-sale, at fair value		1,229,654,000	1,450,731,000
Available-for-sale, Fair Value [Abstract]
Securities available-for-sale, at fair value		1,229,654,000	1,450,731,000
Asset-backed securities
Securities available-for-sale [Abstract]
Amortized Cost		379,107,000	269,941,000
Gross Unrealized Gains		820,000	1,026,000
Gross Unrealized Losses		4,345,000	1,957,000
Securities available-for-sale, at fair value		375,582,000	269,010,000
Available-for-sale, Fair Value [Abstract]
Securities available-for-sale, at fair value		375,582,000	269,010,000
Corporate notes and other
Securities available-for-sale [Abstract]
Amortized Cost		69,399,000	56,062,000
Gross Unrealized Gains		170,000	302,000
Gross Unrealized Losses		2,523,000	1,401,000
Securities available-for-sale, at fair value		67,046,000	54,963,000
Available-for-sale, Fair Value [Abstract]
Securities available-for-sale, at fair value		67,046,000	54,963,000
State and municipal securities
Securities held-to-maturity [Abstract]
Amortized Cost		194,282,000	194,043,000
Gross Unrealized Gains		152,000	4,979,000
Gross Unrealized Losses		1,303,000	12,000
Securities held-to-maturity, fair value		193,131,000	199,010,000
Held-to-maturity, Amortized Cost [Abstract]
Amortized Cost		194,282,000	194,043,000
Held-to-maturity, Fair Value [Abstract]
Fair Value		$ 193,131,000	$ 199,010,000