Investments (Aggregate Values and Unrealized Gains and Losses) (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Investment securities available for sale
Cost	$ 8,884,548	$ 7,052,152
Gross unrealized losses	8,500	17,819
Debt Securities, Available-for-sale, Allowance for Credit Loss	0
Fair value	9,019,788	7,059,674
Investment in marketable equity securities at cost	100,408	59,262
Available-for-sale Equity Securities, Accumulated Gross Unrealized Gain, before Tax	3,353	23,304
Available-for-sale Equity Securities, Accumulated Gross Unrealized Loss, before Tax	10,687	233
Fair value	93,074	82,333
Investment securities held to maturity
Cost	747,732	30,996
Gross unrealized gains	13,555
Gross unrealized losses	35
Allowance for credit loss on debt securities held to maturity	0
Fair value	761,252	30,996
Cost	9,732,688	7,142,410
Gross unrealized gains	160,648	48,645
Gross unrealized losses	19,222	18,052
Debt Securities, Available-for-sale, Held-to-maturity and Equity Securities, Allowance for Credit Losses	0
Fair value	9,874,114	7,173,003
Investment securities available for sale at cost	8,884,548	7,052,152
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	143,740	25,341
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	8,500	17,819
Debt Securities, Available-for-sale	9,019,788	7,059,674
Interest receivable, available-for-sale debt securities	23,600
Interest receivable, held-to-maturity debt securities	1,700
U.S. Treasury
Investment securities available for sale
Debt Securities, Available-for-sale, Allowance for Credit Loss	0
Investment securities held to maturity
Investment securities available for sale at cost	654,588	409,397
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	174	602
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	0	0
Debt Securities, Available-for-sale	654,762	409,999
Government agency
Investment securities available for sale
Gross unrealized losses	4,961	2,241
Debt Securities, Available-for-sale, Allowance for Credit Loss	0
Investment securities held to maturity
Investment securities available for sale at cost	659,260	684,085
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	642	928
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	4,961	2,241
Debt Securities, Available-for-sale	654,941	682,772
Residential Mortgage Backed Securities
Investment securities available for sale
Gross unrealized losses	312	15,387
Debt Securities, Available-for-sale, Allowance for Credit Loss	0
Investment securities held to maturity
Cost	614,489	0
Gross unrealized gains	12,905
Gross unrealized losses	0
Allowance for credit loss on debt securities held to maturity	0
Fair value	627,394
Investment securities available for sale at cost	5,968,192	5,269,060
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	101,788	13,417
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	312	15,387
Debt Securities, Available-for-sale	6,069,668	5,267,090
Commercial Mortgage Backed Securities
Investment securities available for sale
Cost	1,058,640	373,105
Gross unrealized gains	32,004	6,974
Gross unrealized losses	434	59
Debt Securities, Available-for-sale, Allowance for Credit Loss	0
Fair value	1,090,210	380,020
Investment securities held to maturity
Cost	130,987	0
Gross unrealized gains	650
Gross unrealized losses	35
Allowance for credit loss on debt securities held to maturity
Fair value	131,602
Residential mortgage-backed securities
Investment securities held to maturity
Cost		30,996
Gross unrealized gains		0
Gross unrealized losses		0
Fair value		30,996
Commercial mortgage-backed securities
Investment securities available for sale
Gross unrealized losses	2,793
Debt Securities, Available-for-sale, Allowance for Credit Loss	0
Investment securities held to maturity
Investment securities available for sale at cost	543,868	198,278
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	9,132	3,420
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	2,793	132
Debt Securities, Available-for-sale	550,207	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	2,256	30,996
Gross unrealized gains	0	0
Gross unrealized losses	0	0
Allowance for credit loss on debt securities held to maturity	0
Fair value	$ 2,256	$ 30,996

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