Investment Securities - Amortized cost and fair value for available for sale securities (Details) - USD ($)	3 Months Ended	9 Months Ended
Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Investment securities available for sale
Amortized cost	$ 3,498,087,000	$ 1,789,509,000	$ 3,498,087,000	$ 1,789,509,000	$ 1,940,763,000
Gross Unrealized Gains	68,029,000	26,156,000	68,029,000	26,156,000	19,823,000
Gross Unrealized Losses	(4,187,000)	(2,531,000)	(4,187,000)	(2,531,000)	(4,539,000)
Fair Value	3,561,929,000	1,813,134,000	3,561,929,000	1,813,134,000	1,956,047,000
Gross realized gains	469,000		469,000
Gross realized losses	454,000,000		454,000,000
Net realized gain (loss)	15,000	437,000	15,000	2,700,000
Government-sponsored entities debt
Investment securities available for sale
Amortized cost	29,882,000	40,322,000	29,882,000	40,322,000	25,356,000
Gross Unrealized Gains	45,000	645,000	45,000	645,000	585,000
Fair Value	29,927,000	40,967,000	29,927,000	40,967,000	25,941,000
State and municipal obligations
Investment securities available for sale
Amortized cost	446,068,000	183,005,000	446,068,000	183,005,000	204,150,000
Gross Unrealized Gains	13,236,000	5,720,000	13,236,000	5,720,000	5,029,000
Gross Unrealized Losses	(109,000)	(43,000)	(109,000)	(43,000)	(764,000)
Fair Value	459,195,000	188,682,000	459,195,000	188,682,000	208,415,000
Mortgage-backed securities
Investment securities available for sale
Amortized cost	3,008,562,000	1,566,182,000	3,008,562,000	1,566,182,000	1,711,257,000
Gross Unrealized Gains	54,671,000	19,791,000	54,671,000	19,791,000	14,209,000
Gross Unrealized Losses	(4,078,000)	(2,488,000)	(4,078,000)	(2,488,000)	(3,775,000)
Fair Value	3,059,155,000	1,583,485,000	3,059,155,000	1,583,485,000	1,721,691,000
Corporate securities.
Investment securities available for sale
Amortized cost	13,575,000		13,575,000
Gross Unrealized Gains	77,000		77,000
Fair Value	13,652,000		13,652,000
Mortgage-backed securities issued by private label holdings
Investment securities available for sale
Fair Value	$ 0	$ 0	$ 0	0	$ 0
VISA Class B Shares
Investment securities available for sale
Net realized gain (loss)				5,400,000
Securities other than VISA Class B Share
Investment securities available for sale
Net realized gain (loss)				$ 2,700,000

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of realized gain (loss) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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