Investment Securities - Amortized cost and fair value for available for sale securities (Details) - USD ($)	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Investment securities available for sale
Amortized cost	$ 1,915,461,000	$ 1,472,538,000	$ 1,940,763,000
Gross Unrealized Gains	58,641,000	5,602,000	19,823,000
Gross Unrealized Losses	(2,907,000)	(11,891,000)	(4,539,000)
Fair Value	1,971,195,000	1,466,249,000	1,956,047,000
Net realized gain (loss)	0	(541,000)
Government-sponsored entities debt
Investment securities available for sale
Amortized cost	4,882,000	65,567,000	25,356,000
Gross Unrealized Gains	39,000	441,000	585,000
Gross Unrealized Losses		(304,000)
Fair Value	4,921,000	65,704,000	25,941,000
State and municipal obligations
Investment securities available for sale
Amortized cost	215,388,000	190,302,000	204,150,000
Gross Unrealized Gains	6,631,000	3,170,000	5,029,000
Gross Unrealized Losses	(415,000)	(54,000)	(764,000)
Fair Value	221,604,000	193,418,000	208,415,000
Mortgage-backed securities
Investment securities available for sale
Amortized cost	1,695,191,000	1,216,669,000	1,711,257,000
Gross Unrealized Gains	51,971,000	1,991,000	14,209,000
Gross Unrealized Losses	(2,492,000)	(11,533,000)	(3,775,000)
Fair Value	1,744,670,000	1,207,127,000	1,721,691,000
Mortgage-backed securities issued by private label holdings
Investment securities available for sale
Fair Value	$ 0	0	$ 0
VISA Class B Shares
Investment securities available for sale
Net realized gain (loss)		3,500,000
Securities other than VISA Class B Share
Investment securities available for sale
Net realized gain (loss)		$ (3,000,000.0)