Investments (Maturity Information) (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Investment securities available for sale
Total investment securities available for sale, cost	$ 6,911,965	$ 7,052,152
Investment securities available for sale	7,014,243	7,059,674
Investment securities held to maturity
Maturing in one year or less, cost	1,507	30,746
Maturing in one year or less, fair value	1,507	30,746
Maturing in one through five years, cost	749	250
Maturing in one through five years, fair value	749	250
Investment securities held to maturity, cost	2,816,982	30,996
Total investment securities held to maturity, fair value	2,838,499	30,996
Debt Securities
Investment securities available for sale
Maturing in one year or less, cost	500,846	406,325
Maturing in one through five years, cost	72,565	24,496
Maturing in five through ten years, cost	508,320	185,209
Maturing in over ten years, cost	8,971	109,872
Maturing in one year or less, fair value	500,954	406,927
Maturing in one through five years, fair value	73,881	24,971
Maturing in five through ten years, fair value	519,570	187,868
Maturing in over ten years, fair value	8,807	110,026
Government Agency
Investment securities available for sale
Total investment securities available for sale, cost	706,241	684,085
Investment securities available for sale	701,391	682,772
Residential Mortgage Backed Securities
Investment securities available for sale
Total investment securities available for sale, cost	4,369,130	5,269,060
Investment securities available for sale	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, fair value	1,895,381	0
Total investment securities held to maturity, fair value	1,895,381
Commercial Mortgage Backed Securities
Investment securities available for sale
Total investment securities available for sale, cost	745,892	373,105
Investment securities available for sale	771,537	380,020
Investment securities held to maturity
Investment securities held to maturity, cost	937,034	0
Investment securities held to maturity, fair value	940,862	$ 0
Total investment securities held to maturity, fair value	$ 940,862