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Investment Securities - Amortized cost and fair value for available for sale securities (Details) - USD ($)	3 Months Ended				9 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Investment securities available for sale
Amortized cost	$ 1,789,509,000			$ 1,598,898,000	$ 1,789,509,000	$ 1,598,898,000	$ 1,540,650,000
Gross Unrealized Gains	26,156,000			1,229,000	26,156,000	1,229,000	2,427,000
Gross Unrealized Losses	(2,531,000)			(48,846,000)	(2,531,000)	(48,846,000)	(26,010,000)
Fair Value	1,813,134,000			1,551,281,000	1,813,134,000	1,551,281,000	1,517,067,000
Net realized gain	437,000				2,700,000
Net realized loss				11,000		652,000
Government-sponsored entities debt
Investment securities available for sale
Amortized cost	40,322,000			54,397,000	40,322,000	54,397,000	48,982,000
Gross Unrealized Gains	645,000				645,000		21,000
Gross Unrealized Losses				(1,852,000)		(1,852,000)	(752,000)
Fair Value	40,967,000			52,545,000	40,967,000	52,545,000	48,251,000
State and municipal obligations
Investment securities available for sale
Amortized cost	183,005,000			207,723,000	183,005,000	207,723,000	200,184,000
Gross Unrealized Gains	5,720,000			1,114,000	5,720,000	1,114,000	1,709,000
Gross Unrealized Losses	(43,000)			(2,613,000)	(43,000)	(2,613,000)	(1,125,000)
Fair Value	188,682,000			206,224,000	188,682,000	206,224,000	200,768,000
Mortgage-backed securities
Investment securities available for sale
Amortized cost	1,566,182,000			1,336,778,000	1,566,182,000	1,336,778,000	1,291,484,000
Gross Unrealized Gains	19,791,000			115,000	19,791,000	115,000	697,000
Gross Unrealized Losses	(2,488,000)			(44,381,000)	(2,488,000)	(44,381,000)	(24,133,000)
Fair Value	1,583,485,000			1,292,512,000	1,583,485,000	1,292,512,000	1,268,048,000
VISA Class B Shares
Investment securities available for sale
Net realized gain		$ 5,400,000	$ 5,400,000
Securities other than VISA Class B Share
Investment securities available for sale
Net realized loss					2,700,000
Mortgage-backed securities issued by private label holdings
Investment securities available for sale
Fair Value	$ 0			$ 0	$ 0	$ 0	$ 0