Securities - Amortized Cost and Fair Value of Securities (Details) - USD ($)	Sep. 30, 2020	Jan. 01, 2020	Dec. 31, 2019
Securities available-for-sale [Abstract]
Amortized Cost	$ 3,394,673,000		$ 3,491,633,000
Gross Unrealized Gains	103,496,000		66,147,000
Gross Unrealized Losses	34,747,000		17,785,000
Securities available-for-sale, at fair value	3,463,422,000		3,539,995,000
Available-for-sale, Amortized Cost [Abstract]
Due in one year or less	85,090,000
Due in one year to five years	12,059,000
Due in five years to ten years	175,812,000
Due after ten years	1,317,843,000
Mortgage-backed securities	1,634,329,000
Asset-backed securities	169,540,000
Amortized Cost	3,394,673,000
Available-for-sale, Fair Value [Abstract]
Due in one year or less	85,120,000
Due in one year to five years	12,446,000
Due in five years to ten years	181,102,000
Due after ten years	1,314,703,000
Mortgage-backed securities	1,701,212,000
Asset-backed securities	168,839,000
Securities available-for-sale, at fair value	3,463,422,000		3,539,995,000
Securities held-to-maturity [Abstract]
Amortized Cost	1,039,841,000		188,996,000
Allowance for credit losses - securities held-to-maturity	(191,000)	$ (10,000)	0
HeldToMaturitySecurities, net of allowance for credit losses	1,039,650,000		188,996,000
Gross Unrealized Gains	23,814,000		12,221,000
Gross Unrealized Losses	2,808,000		0
Securities held-to-maturity, fair value	1,060,847,000		201,217,000
Held-to-maturity, Amortized Cost [Abstract]
Due in one year or less	0
Due in one year to five years	1,409,000
Due in five years to ten years	7,209,000
Due after ten years	1,031,223,000
Mortgage-backed securities	0
Asset-backed securities	0
Amortized Cost	1,039,841,000		188,996,000
Held-to-maturity, Fair Value [Abstract]
Due in one year or less	0
Due in one year to five years	1,482,000
Due in five years to ten years	7,293,000
Due after ten years	1,052,072,000
Mortgage-backed securities	0
Asset-backed securities	0
Fair Value	1,060,847,000		201,217,000
U.S. Treasury securities
Securities available-for-sale [Abstract]
Amortized Cost	84,438,000		72,862,000
Gross Unrealized Gains	28,000		19,000
Gross Unrealized Losses	2,000		14,000
Securities available-for-sale, at fair value	84,464,000		72,867,000
Available-for-sale, Fair Value [Abstract]
Securities available-for-sale, at fair value	84,464,000		72,867,000
U.S. government agency securities
Securities available-for-sale [Abstract]
Amortized Cost	75,345,000		80,096,000
Gross Unrealized Gains	1,543,000		306,000
Gross Unrealized Losses	29,000		710,000
Securities available-for-sale, at fair value	76,859,000		79,692,000
Available-for-sale, Fair Value [Abstract]
Securities available-for-sale, at fair value	76,859,000		79,692,000
Mortgage-backed securities
Securities available-for-sale [Abstract]
Amortized Cost	1,634,329,000		1,458,894,000
Gross Unrealized Gains	69,311,000		12,789,000
Gross Unrealized Losses	2,428,000		7,776,000
Securities available-for-sale, at fair value	1,701,212,000		1,463,907,000
Available-for-sale, Fair Value [Abstract]
Securities available-for-sale, at fair value	1,701,212,000		1,463,907,000
State and municipal securities
Securities available-for-sale [Abstract]
Amortized Cost	1,324,383,000		1,669,606,000
Gross Unrealized Gains	31,081,000		52,096,000
Gross Unrealized Losses	27,743,000		7,249,000
Securities available-for-sale, at fair value	1,327,721,000		1,714,453,000
Available-for-sale, Fair Value [Abstract]
Securities available-for-sale, at fair value	1,327,721,000		1,714,453,000
Securities held-to-maturity [Abstract]
Amortized Cost	1,039,841,000		188,996,000
Gross Unrealized Gains	23,814,000		12,221,000
Gross Unrealized Losses	2,808,000		0
Securities held-to-maturity, fair value	1,060,847,000		201,217,000
Held-to-maturity, Amortized Cost [Abstract]
Amortized Cost	1,039,841,000		188,996,000
Held-to-maturity, Fair Value [Abstract]
Fair Value	1,060,847,000		201,217,000
Asset-backed securities
Securities available-for-sale [Abstract]
Amortized Cost	169,540,000		153,963,000
Gross Unrealized Gains	470,000		302,000
Gross Unrealized Losses	1,171,000		1,293,000
Securities available-for-sale, at fair value	168,839,000		152,972,000
Available-for-sale, Fair Value [Abstract]
Securities available-for-sale, at fair value	168,839,000		152,972,000
Corporate notes and other
Securities available-for-sale [Abstract]
Amortized Cost	106,638,000		56,212,000
Gross Unrealized Gains	1,063,000		635,000
Gross Unrealized Losses	3,374,000		743,000
Securities available-for-sale, at fair value	104,327,000		56,104,000
Available-for-sale, Fair Value [Abstract]
Securities available-for-sale, at fair value	$ 104,327,000		$ 56,104,000