Investments (Maturity Information) (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Investment securities held to maturity
Investment securities held to maturity at cost	$ 3,381,078	$ 2,816,982
Investment securities held to maturity at fair value	3,353,078	2,838,499
Investment securities available for sale at cost	7,345,143	6,911,965
Debt Securities, Available-for-sale	7,371,129	7,014,243
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, within One Year, Amortized Cost	1,259	1,507
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, within One Year, Fair Value	1,259	1,507
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Amortized Cost	749	749
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Fair Value	749	749
Debt Securities
Investment securities available for sale
Maturing in one year or less, cost	1,204	500,846
Maturing in one through five years, cost	37,248	72,565
Maturing in five through ten years, cost	545,754	508,320
Maturing in over ten years, cost	16,700	8,971
Maturing in one year or less, fair value	1,208	500,954
Maturing in one through five years, fair value	38,540	73,881
Maturing in five through ten yeas, fair value	568,692	519,570
Maturing in over ten years, fair value	16,663	8,807
Agency Securities
Investment securities held to maturity
Investment securities available for sale at cost	851,860	706,241
Debt Securities, Available-for-sale	853,728	701,391
Residential Mortgage Backed Securities
Investment securities held to maturity
Investment securities held to maturity at cost	2,122,299	1,877,692
Commercial mortgage-backed securities	2,112,561	1,895,381
Investment securities held to maturity at 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	4,801,646	4,438,103
Commercial Mortgage Backed Securities
Investment securities held to maturity
Investment securities held to maturity at cost	1,256,771	937,034
Commercial mortgage-backed securities	1,238,509	940,862
Investment securities held to maturity at fair value	1,238,509	940,862
Investment securities available for sale at cost	1,092,183	745,892
Debt Securities, Available-for-sale	$ 1,090,652	771,537
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