Investments (Aggregate Values and Unrealized Gains and Losses) (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Investment securities available for sale
Investment securities available for sale, cost	$ 6,911,965	$ 7,052,152
Investment securities available for sale, gross unrealized gains	111,189	25,341
Investment securities available for sale, gross unrealized losses	8,911	17,819
Allowance for credit losses	0
Investment securities available for sale, fair value	7,014,243	7,059,674
Investment in marketable equity securities, cost	84,837	59,262
Investment in marketable equity securities, gross unrealized gains	8,654	23,304
Investment in marketable equity securities, gross unrealized losses	1,811	233
Investment in marketable equity securities, fair value	91,680	82,333
Investment securities held to maturity
Investment securities held to maturity, cost	2,816,982	30,996
Investment securities held to maturity, gross unrealied gains	21,573
Investment securities held to maturity, gross unrealized losses	56
Debt securities, held-to-maturity, allowance for credit loss	0
Investment securities held to maturity, fair value	2,838,499	30,996
Total investment securities, cost	9,813,784	7,142,410
Total investment securities, gross unrealized gains	141,416	48,645
Total investment securities, fair value	10,778	18,052
Debt Securities, Available-for-sale, Held-to-maturity and Equity Securities, Allowance for Credit Losses	0
Total investment securities, gross unrealized losses	9,944,422	7,173,003
U. S. Treasury
Investment securities available for sale
Investment securities available for sale, cost	499,832	409,397
Investment securities available for sale, gross unrealized gains	101	602
Investment securities available for sale, gross unrealized losses	0	0
Allowance for credit losses	0
Investment securities available for sale, fair value	499,933	409,999
Government Agency
Investment securities available for sale
Investment securities available for sale, cost	706,241	684,085
Investment securities available for sale, gross unrealized gains	723	928
Investment securities available for sale, gross unrealized losses	5,573	2,241
Allowance for credit losses	0
Investment securities available for sale, fair value	701,391	682,772
Residential Mortgage Backed Securities
Investment securities available for sale
Investment securities available for sale, cost	4,369,130	5,269,060
Investment securities available for sale, gross unrealized gains	70,283	13,417
Investment securities available for sale, gross unrealized losses	1,310	15,387
Allowance for credit losses	0
Investment securities available for sale, fair value	4,438,103	5,267,090
Investment securities held to maturity
Investment securities held to maturity, cost	1,877,692	0
Investment securities held to maturity, gross unrealied gains	17,689
Investment securities held to maturity, gross unrealized losses	0
Debt securities, held-to-maturity, allowance for credit loss	0
Investment securities held to maturity, fair value	1,895,381
Commercial Mortgage Backed Securities
Investment securities available for sale
Investment securities available for sale, cost	745,892	373,105
Investment securities available for sale, gross unrealized gains	25,645	6,974
Investment securities available for sale, gross unrealized losses	0	59
Allowance for credit losses	0
Investment securities available for sale, fair value	771,537	380,020
Investment securities held to maturity
Investment securities held to maturity, cost	937,034	0
Investment securities held to maturity, gross unrealied gains	3,884
Investment securities held to maturity, gross unrealized losses	56
Debt securities, held-to-maturity, allowance for credit loss	0
Investment securities held to maturity, fair value	940,862
Corporate Bonds
Investment securities available for sale
Investment securities available for sale, cost	590,870	198,278
Investment securities available for sale, gross unrealized gains	14,437	3,420
Investment securities available for sale, gross unrealized losses	2,028	132
Allowance for credit losses	0
Investment securities available for sale, fair value	603,279	201,566
US States and Political Subdivisions Debt Securities
Investment securities available for sale
Investment securities available for sale, cost		118,227
Investment securities available for sale, gross unrealized gains		0
Investment securities available for sale, gross unrealized losses		0
Investment securities available for sale, fair value		118,227
Other Debt Obligations
Investment securities held to maturity
Investment securities held to maturity, cost	2,256	30,996
Investment securities held to maturity, gross unrealied gains	0	0
Investment securities held to maturity, gross unrealized losses	0	0
Debt securities, held-to-maturity, allowance for credit loss	0
Investment securities held to maturity, fair value	$ 2,256	$ 30,996

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