Investments (Aggregate Values and Unrealized Gains and Losses) (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Investment securities available for sale
Cost	$ 7,656,464	$ 7,052,152
Gross unrealized losses	11,483	17,819
Debt Securities, Available-for-sale, Allowance for Credit Loss	0
Fair value	7,789,034	7,059,674
Cost	344,161	59,262
Available-for-sale Equity Securities, Accumulated Gross Unrealized Gain, before Tax	4,266	23,304
Available-for-sale Equity Securities, Accumulated Gross Unrealized Loss, before Tax	32,926	233
Fair value	315,501	82,333
Investment securities held to maturity
Cost	740,662	30,996
Gross unrealized gains	12,538
Gross unrealized losses	22
Debt Securities, Held-to-maturity, Allowance for Credit Loss	0
Fair value	753,178	30,996
Cost	8,741,287	7,142,410
Gross unrealized gains	160,857	48,645
Gross unrealized losses	44,431	18,052
Debt Securities, Available-for-sale, Held-to-maturity and Equity Securities, Allowance for Credit Losses	0
Fair value	8,857,713	7,173,003
Debt Securities, Available-for-sale, Amortized Cost	7,656,464	7,052,152
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	144,053	25,341
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	11,483	17,819
Debt Securities, Available-for-sale	7,789,034	7,059,674
U.S. Treasury
Investment securities available for sale
Debt Securities, Available-for-sale, Allowance for Credit Loss	0
Investment securities held to maturity
Debt Securities, Available-for-sale, Amortized Cost	204,971	409,397
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	811	602
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	0	0
Debt Securities, Available-for-sale	205,782	409,999
Government agency
Investment securities available for sale
Gross unrealized losses	2,916	2,241
Debt Securities, Available-for-sale, Allowance for Credit Loss	0
Investment securities held to maturity
Debt Securities, Available-for-sale, Amortized Cost	762,342	684,085
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	1,005	928
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	2,916	2,241
Debt Securities, Available-for-sale	760,431	682,772
Residential Mortgage Backed Securities
Investment securities available for sale
Gross unrealized losses	2,791	15,387
Debt Securities, Available-for-sale, Allowance for Credit Loss	0
Investment securities held to maturity
Cost	720,441	0
Gross unrealized losses	22
Debt Securities, Available-for-sale, Amortized Cost	5,832,518	5,269,060
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	123,114	13,417
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	2,791	15,387
Debt Securities, Available-for-sale	5,952,841	5,267,090
Commercial Mortgage Backed Securities [Member]
Investment securities available for sale
Cost	635,565	373,105
Gross unrealized gains	15,603	6,974
Gross unrealized losses	603	59
Debt Securities, Available-for-sale, Allowance for Credit Loss	0
Fair value	650,565	380,020
Residential mortgage-backed securities
Investment securities held to maturity
Cost	720,441	30,996
Gross unrealized gains	12,538	0
Gross unrealized losses	22	0
Debt Securities, Held-to-maturity, Allowance for Credit Loss	0
Fair value	732,957	30,996
Commercial mortgage-backed securities
Investment securities available for sale
Gross unrealized losses	5,173
Debt Securities, Available-for-sale, Allowance for Credit Loss	0
Investment securities held to maturity
Debt Securities, Available-for-sale, Amortized Cost	221,068	198,278
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	3,520	3,420
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	5,173	132
Debt Securities, Available-for-sale	219,415	201,566
US States and Political Subdivisions Debt Securities [Member]
Investment securities available for sale
Cost		118,227
Gross unrealized gains		0
Gross unrealized losses		0
Fair value		118,227
Other
Investment securities held to maturity
Cost	20,221	30,996
Gross unrealized gains	0	0
Gross unrealized losses	0	0
Debt Securities, Held-to-maturity, Allowance for Credit Loss	0
Fair value	$ 20,221	$ 30,996

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