Investments (Aggregate Values and Unrealized Gains and Losses) (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Investment securities available for sale
Investment in marketable equity securities at cost	$ 85,554	$ 84,837
Investment in marketable equity securities, gross unrealized gains	37,744	8,654
Investment in marketable equity securities, gross unrealized losses	151	1,811
Fair value	123,147	91,680
Investment securities held to maturity
Debt Securities, Held-to-maturity, Total	3,381,078	2,816,982
Gross unrealized gains	5,316
Gross unrealized losses	33,316
Fair value	3,353,078	2,838,499
Debt Securities, Available-for-sale, Held-to-maturity and Equity Securities, Cost	10,811,775	9,813,784
Gross unrealized gains	102,797	141,416
Gross unrealized losses	67,218	10,778
Fair value	10,847,354	9,944,422
Investment securities available for sale at cost	7,345,143	6,911,965
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	59,737	111,189
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	33,751	8,911
Debt Securities, Available-for-sale	7,371,129	7,014,243
Interest receivable, available-for-sale debt securities	21,100	17,600
Interest receivable, held-to-maturity debt securities	6,000	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 held to maturity
Investment securities available for sale at cost	851,860	706,241
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	5,348	723
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	3,480	5,573
Debt Securities, Available-for-sale	853,728	701,391
Residential Mortgage Backed Securities
Investment securities held to maturity
Debt Securities, Held-to-maturity, Total	2,122,299	1,877,692
Gross unrealized gains	5,313	17,689
Gross unrealized losses	15,051	0
Fair value	2,112,561	1,895,381
Investment securities available for sale at cost	4,800,194	4,369,130
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	19,219	70,283
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	17,767	1,310
Debt Securities, Available-for-sale	4,801,646	4,438,103
Commercial Mortgage Backed Securities
Investment securities held to maturity
Debt Securities, Held-to-maturity, Total	1,256,771	937,034
Gross unrealized gains	3	3,884
Gross unrealized losses	18,265	56
Fair value	1,238,509	940,862
Investment securities available for sale at cost	1,092,183	745,892
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	9,875	25,645
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	11,406	0
Debt Securities, Available-for-sale	1,090,652	771,537
Residential mortgage-backed securities
Investment securities held to maturity
Debt Securities, Held-to-maturity, Total		2,816,982
Gross unrealized gains		21,573
Gross unrealized losses		56
Fair value		2,838,499
Commercial mortgage-backed securities
Investment securities held to maturity
Investment securities available for sale at cost	600,906	590,870
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	25,295	14,437
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	1,098	2,028
Debt Securities, Available-for-sale	625,103	603,279
Other
Investment securities held to maturity
Debt Securities, Held-to-maturity, Total	2,008	2,256
Gross unrealized gains	0	0
Gross unrealized losses	0	0
Fair value	$ 2,008	$ 2,256