Investments (Aggregate Values and Unrealized Gains and Losses) (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Investment securities available for sale
Amortized cost	$ 7,270,552	$ 6,911,965
Gross unrealized losses	44,888	8,911
Fair value	7,307,122	7,014,243
Investment in marketable equity securities at cost	84,297	84,837
Available-for-sale Equity Securities, Accumulated Gross Unrealized Gain, before Tax	22,872	8,654
Available-for-sale Equity Securities, Accumulated Gross Unrealized Loss, before Tax	283	1,811
Fair value	106,886	91,680
Investment securities held to maturity
Amortized cost	2,808,099	2,816,982
Gross unrealized gains	5,204
Gross unrealized losses	38,321
Fair value	2,774,982	2,838,499
Amortized cost	10,162,948	9,813,784
Gross unrealized gains	109,534	141,416
Gross unrealized losses	83,492	10,778
Fair value	10,188,990	9,944,422
Investment securities available for sale at cost	7,270,552	6,911,965
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	81,458	111,189
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	44,888	8,911
Debt Securities, Available-for-sale	7,307,122	7,014,243
Interest receivable, available-for-sale debt securities	21,300	17,600
Interest receivable, held-to-maturity debt securities	5,300	5,400
U.S. Treasury
Investment securities held to maturity
Investment securities available for sale at cost		499,832
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax		101
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax		0
Debt Securities, Available-for-sale		499,933
Government agency
Investment securities available for sale
Gross unrealized losses	6,403	5,573
Investment securities held to maturity
Investment securities available for sale at cost	852,305	706,241
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	784	723
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	6,403	5,573
Debt Securities, Available-for-sale	846,686	701,391
Residential Mortgage Backed Securities
Investment securities available for sale
Gross unrealized losses	26,046	1,310
Investment securities held to maturity
Amortized cost	1,740,314	1,877,692
Gross unrealized gains	5,191	17,689
Gross unrealized losses	20,510	0
Fair value	1,724,995	1,895,381
Investment securities available for sale at cost	4,736,893	4,369,130
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	47,742	70,283
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	26,046	1,310
Debt Securities, Available-for-sale	4,758,589	4,438,103
Commercial Mortgage Backed Securities
Investment securities available for sale
Amortized cost	1,096,259	745,892
Gross unrealized gains	14,619	25,645
Gross unrealized losses	11,891	0
Fair value	1,098,987	771,537
Investment securities held to maturity
Amortized cost	1,065,778	937,034
Gross unrealized gains	13	3,884
Gross unrealized losses	17,811	56
Fair value	1,047,980	940,862
Residential mortgage-backed securities
Investment securities held to maturity
Amortized cost		2,816,982
Gross unrealized gains		21,573
Gross unrealized losses		56
Fair value		2,838,499
Commercial mortgage-backed securities
Investment securities available for sale
Gross unrealized losses	548
Investment securities held to maturity
Investment securities available for sale at cost	585,095	590,870
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	18,313	14,437
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	548	2,028
Debt Securities, Available-for-sale	602,860	603,279
Other
Investment securities held to maturity
Amortized cost	2,007	2,256
Gross unrealized gains	0	0
Gross unrealized losses	0	0
Fair value	$ 2,007	$ 2,256

Debt Securities, Available-for-sale, Held-to-maturity and Equity Securities

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