Investment Securities - Amortized cost and fair value for available for sale securities (Details) - USD ($)	3 Months Ended	6 Months Ended
Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Investment securities available for sale
Amortized cost	$ 3,075,776,000	$ 1,700,681,000	$ 3,075,776,000	$ 1,700,681,000	$ 1,940,763,000
Gross Unrealized Gains	64,839,000	19,530,000	64,839,000	19,530,000	19,823,000
Gross Unrealized Losses	(2,897,000)	(2,935,000)	(2,897,000)	(2,935,000)	(4,539,000)
Fair Value	3,137,718,000	1,717,276,000	3,137,718,000	1,717,276,000	1,956,047,000
Net realized gain (loss)	0	1,700,000	0	2,300,000
Government-sponsored entities debt
Investment securities available for sale
Amortized cost	4,882,000	65,604,000	4,882,000	65,604,000	25,356,000
Gross Unrealized Gains	31,000	933,000	31,000	933,000	585,000
Gross Unrealized Losses		(8,000)		(8,000)
Fair Value	4,913,000	66,529,000	4,913,000	66,529,000	25,941,000
State and municipal obligations
Investment securities available for sale
Amortized cost	443,732,000	177,533,000	443,732,000	177,533,000	204,150,000
Gross Unrealized Gains	10,487,000	4,614,000	10,487,000	4,614,000	5,029,000
Gross Unrealized Losses	(565,000)	(1,000)	(565,000)	(1,000)	(764,000)
Fair Value	453,654,000	182,146,000	453,654,000	182,146,000	208,415,000
Mortgage-backed securities
Investment securities available for sale
Amortized cost	2,421,586,000	1,457,544,000	2,421,586,000	1,457,544,000	1,711,257,000
Gross Unrealized Gains	54,214,000	13,983,000	54,214,000	13,983,000	14,209,000
Gross Unrealized Losses	(2,331,000)	(2,926,000)	(2,331,000)	(2,926,000)	(3,775,000)
Fair Value	2,473,469,000	1,468,601,000	2,473,469,000	1,468,601,000	1,721,691,000
U.S. Treasuries
Investment securities available for sale
Amortized cost	199,988,000		199,988,000
Gross Unrealized Losses	(1,000)		(1,000)
Fair Value	199,987,000		199,987,000
Corporate securities.
Investment securities available for sale
Amortized cost	5,588,000		5,588,000
Gross Unrealized Gains	107,000		107,000
Fair Value	5,695,000		5,695,000
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)				$ 3,100,000

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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