Investments (Maturity Information) (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Investment securities available for sale
Total investment securities available for sale, cost	$ 7,656,464	$ 7,052,152
Total investment securities available for sale, fair value	7,789,034	7,059,674
Investment securities held to maturity
Investment securities held to maturity	740,662	30,996
Fair Value	753,178	30,996
Debt Securities, Available-for-sale, Amortized Cost	7,656,464	7,052,152
Debt Securities, Available-for-sale	7,789,034	7,059,674
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, within One Year, Amortized Cost	19,472	30,746
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, within One Year, Fair Value	19,472	30,746
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Amortized Cost	749	250
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Fair Value	749	250
Debt Securities
Investment securities available for sale
Maturing in one year or less, cost	204,971	406,325
Maturing in one through five years, cost	32,639	24,496
Maturing in five through ten years, cost	184,476	185,209
Maturing in over ten years, cost	3,953	109,872
Maturing in one year or less, fair value	205,782	406,927
Maturing in one through five years, fair value	32,446	24,971
Maturing in five through ten yeas, fair value	183,469	187,868
Maturing in over ten years, fair value	3,500	110,026
Agency Securities
Investment securities held to maturity
Debt Securities, Available-for-sale, Amortized Cost	762,342	684,085
Debt Securities, Available-for-sale	760,431	682,772
Residential Mortgage Backed Securities
Investment securities held to maturity
Investment securities held to maturity	720,441	0
Commercial mortgage-backed securities	732,957	0
Debt Securities, Available-for-sale, Amortized Cost	5,832,518	5,269,060
Debt Securities, Available-for-sale	5,952,841	5,267,090
Commercial Mortgage Backed Securities [Member]
Investment securities available for sale
Total investment securities available for sale, cost	635,565	373,105
Total investment securities available for sale, fair value	650,565	380,020
Residential mortgage-backed securities
Investment securities held to maturity
Investment securities held to maturity	720,441	30,996
Fair Value	$ 732,957	$ 30,996