Securities - Amortized Cost and Fair Value of Securities (Details) - USD ($)	Mar. 31, 2020	Jan. 01, 2020	Dec. 31, 2019
Securities available-for-sale [Abstract]
Amortized Cost	$ 3,009,748,000		$ 3,491,633,000
Gross Unrealized Gains	82,993,000		66,147,000
Gross Unrealized Losses	62,177,000		17,785,000
Securities available-for-sale, at fair value	3,030,564,000		3,539,995,000
Available-for-sale, Amortized Cost [Abstract]
Due in one year or less	73,838,000
Due in one year to five years	10,770,000
Due in five years to ten years	149,431,000
Due after ten years	1,057,281,000
Mortgage-backed securities	1,558,424,000
Asset-backed securities	160,004,000
Amortized Cost	3,009,748,000
Available-for-sale, Fair Value [Abstract]
Due in one year or less	74,226,000
Due in one year to five years	10,946,000
Due in five years to ten years	148,673,000
Due after ten years	1,031,679,000
Mortgage-backed securities	1,618,580,000
Asset-backed securities	146,460,000
Securities available-for-sale, at fair value	3,030,564,000		3,539,995,000
Securities held-to-maturity [Abstract]
Amortized Cost	1,059,405,000		188,996,000
Allowance for credit losses - securities held-to-maturity	(148,000)	$ (10,000)
HeldToMaturitySecurities, net of allowance for credit losses	1,059,257,000		188,996,000
Gross Unrealized Gains	14,019,000		12,221,000
Gross Unrealized Losses	22,333,000		0
Securities held-to-maturity, fair value	1,051,091,000		201,217,000
Held-to-maturity, Amortized Cost [Abstract]
Due in one year or less	1,055,000
Due in one year to five years	0
Due in five years to ten years	5,768,000
Due after ten years	1,052,582,000
Mortgage-backed securities	0
Asset-backed securities	0
Amortized Cost	1,059,405,000		188,996,000
Held-to-maturity, Fair Value [Abstract]
Due in one year or less	1,055,000
Due in one year to five years	0
Due in five years to ten years	5,863,000
Due after ten years	1,044,173,000
Mortgage-backed securities	0
Asset-backed securities	0
Fair Value	1,051,091,000		201,217,000
U.S. Treasury securities
Securities available-for-sale [Abstract]
Amortized Cost	72,809,000		72,862,000
Gross Unrealized Gains	391,000		19,000
Gross Unrealized Losses	0		14,000
Securities available-for-sale, at fair value	73,200,000		72,867,000
Available-for-sale, Fair Value [Abstract]
Securities available-for-sale, at fair value	73,200,000		72,867,000
U.S. government agency securities
Securities available-for-sale [Abstract]
Amortized Cost	92,162,000		80,096,000
Gross Unrealized Gains	1,237,000		306,000
Gross Unrealized Losses	1,072,000		710,000
Securities available-for-sale, at fair value	92,327,000		79,692,000
Available-for-sale, Fair Value [Abstract]
Securities available-for-sale, at fair value	92,327,000		79,692,000
Mortgage-backed securities
Securities available-for-sale [Abstract]
Amortized Cost	1,558,424,000		1,458,894,000
Gross Unrealized Gains	63,092,000		12,789,000
Gross Unrealized Losses	2,936,000		7,776,000
Securities available-for-sale, at fair value	1,618,580,000		1,463,907,000
Available-for-sale, Fair Value [Abstract]
Securities available-for-sale, at fair value	1,618,580,000		1,463,907,000
State and municipal securities
Securities available-for-sale [Abstract]
Amortized Cost	1,060,142,000		1,669,606,000
Gross Unrealized Gains	17,916,000		52,096,000
Gross Unrealized Losses	41,439,000		7,249,000
Securities available-for-sale, at fair value	1,036,619,000		1,714,453,000
Available-for-sale, Fair Value [Abstract]
Securities available-for-sale, at fair value	1,036,619,000		1,714,453,000
Securities held-to-maturity [Abstract]
Amortized Cost	1,059,405,000		188,996,000
Gross Unrealized Gains	14,019,000		12,221,000
Gross Unrealized Losses	22,333,000		0
Securities held-to-maturity, fair value	1,051,091,000		201,217,000
Held-to-maturity, Amortized Cost [Abstract]
Amortized Cost	1,059,405,000		188,996,000
Held-to-maturity, Fair Value [Abstract]
Fair Value	1,051,091,000		201,217,000
Asset-backed securities
Securities available-for-sale [Abstract]
Amortized Cost	160,004,000		153,963,000
Gross Unrealized Gains	47,000		302,000
Gross Unrealized Losses	13,591,000		1,293,000
Securities available-for-sale, at fair value	146,460,000		152,972,000
Available-for-sale, Fair Value [Abstract]
Securities available-for-sale, at fair value	146,460,000		152,972,000
Corporate notes and other
Securities available-for-sale [Abstract]
Amortized Cost	66,207,000		56,212,000
Gross Unrealized Gains	310,000		635,000
Gross Unrealized Losses	3,139,000		743,000
Securities available-for-sale, at fair value	63,378,000		56,104,000
Available-for-sale, Fair Value [Abstract]
Securities available-for-sale, at fair value	$ 63,378,000		$ 56,104,000