Investments (Maturity Information) (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Investment securities available for sale
Total investment securities available for sale, cost	$ 7,270,552	$ 6,911,965
Total investment securities available for sale, fair value	7,307,122	7,014,243
Investment securities held to maturity
Investment securities held to maturity at cost	2,808,099	2,816,982
Investment securities held to maturity at fair value	2,774,982	2,838,499
Investment securities available for sale at cost	7,270,552	6,911,965
Debt Securities, Available-for-sale	7,307,122	7,014,243
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, within One Year, Amortized Cost	998	1,507
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, within One Year, Fair Value	998	1,507
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Amortized Cost	1,009	749
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Fair Value	1,009	749
Debt Securities
Investment securities available for sale
Maturing in one year or less, cost	1,020	500,846
Maturing in one through five years, cost	72,340	72,565
Maturing in five through ten years, cost	502,757	508,320
Maturing in over ten years, cost	8,978	8,971
Maturing in one year or less, fair value	1,021	500,954
Maturing in one through five years, fair value	74,536	73,881
Maturing in five through ten yeas, fair value	517,922	519,570
Maturing in over ten years, fair value	9,381	8,807
Agency Securities
Investment securities held to maturity
Investment securities available for sale at cost	852,305	706,241
Debt Securities, Available-for-sale	846,686	701,391
Residential Mortgage Backed Securities
Investment securities held to maturity
Investment securities held to maturity at cost	1,740,314	1,877,692
Commercial mortgage-backed securities	1,724,995	1,895,381
Investment securities held to maturity at 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	4,758,589	4,438,103
Commercial Mortgage Backed Securities
Investment securities available for sale
Total investment securities available for sale, cost	1,096,259	745,892
Total investment securities available for sale, fair value	1,098,987	771,537
Investment securities held to maturity
Investment securities held to maturity at cost	1,065,778	937,034
Commercial mortgage-backed securities	1,047,980	940,862
Investment securities held to maturity at fair value	$ 1,047,980	940,862
Residential mortgage-backed securities
Investment securities held to maturity
Investment securities held to maturity at cost		2,816,982
Investment securities held to maturity at fair value		$ 2,838,499