Investments (Aggregate Values and Unrealized Gains and Losses) (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Investment securities available for sale
Cost	$ 8,472,398	$ 7,052,152
Gross unrealized losses	10,187	17,819
Debt Securities, Available-for-sale, Allowance for Credit Loss	0
Fair value	8,619,282	7,059,674
Investment in marketable equity securities at cost	116,322	59,262
Available-for-sale Equity Securities, Accumulated Gross Unrealized Gain, before Tax	6,297	23,304
Available-for-sale Equity Securities, Accumulated Gross Unrealized Loss, before Tax	8,303	233
Fair value	114,316	82,333
Investment securities held to maturity
Cost	774,878	30,996
Gross unrealized gains	16,447
Gross unrealized losses	0
Allowance for credit loss on debt securities held to maturity	0
Fair value	791,325	30,996
Cost	9,363,598	7,142,410
Gross unrealized gains	179,815	48,645
Gross unrealized losses	18,490	18,052
Debt Securities, Available-for-sale, Held-to-maturity and Equity Securities, Allowance for Credit Losses	0
Fair value	9,524,923	7,173,003
Investment securities available for sale at cost	8,472,398	7,052,152
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	157,071	25,341
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	10,187	17,819
Debt Securities, Available-for-sale	8,619,282	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
Investment securities available for sale at cost	754,350	409,397
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	236	602
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	0	0
Debt Securities, Available-for-sale	754,586	409,999
Government agency
Investment securities available for sale
Gross unrealized losses	4,256	2,241
Debt Securities, Available-for-sale, Allowance for Credit Loss	0
Investment securities held to maturity
Investment securities available for sale at cost	549,673	684,085
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	292	928
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	4,256	2,241
Debt Securities, Available-for-sale	545,709	682,772
Residential Mortgage Backed Securities
Investment securities available for sale
Gross unrealized losses	694	15,387
Debt Securities, Available-for-sale, Allowance for Credit Loss	0
Investment securities held to maturity
Cost	693,143	0
Gross unrealized gains	16,196
Gross unrealized losses	0
Allowance for credit loss on debt securities held to maturity	0
Fair value	709,339
Investment securities available for sale at cost	5,671,151	5,269,060
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	119,935	13,417
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	694	15,387
Debt Securities, Available-for-sale	5,790,392	5,267,090
Commercial Mortgage Backed Securities
Investment securities available for sale
Cost	1,047,601	373,105
Gross unrealized gains	32,221	6,974
Gross unrealized losses	0	59
Debt Securities, Available-for-sale, Allowance for Credit Loss	0
Fair value	1,079,822	380,020
Investment securities held to maturity
Cost	78,729	0
Gross unrealized gains	251
Gross unrealized losses	0
Allowance for credit loss on debt securities held to maturity
Fair value	78,980
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	5,237
Debt Securities, Available-for-sale, Allowance for Credit Loss	0
Investment securities held to maturity
Investment securities available for sale at cost	449,623	198,278
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	4,387	3,420
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	5,237	132
Debt Securities, Available-for-sale	448,773	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	3,006	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	$ 3,006	$ 30,996

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