Investments (Maturity Information) (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Investment securities available for sale
Total investment securities available for sale, cost	$ 8,884,548	$ 7,052,152
Total investment securities available for sale, fair value	9,019,788	7,059,674
Investment securities held to maturity
Investment securities held to maturity at cost	747,732	30,996
Investment securities held to maturity at fair value	761,252	30,996
Investment securities available for sale at cost	8,884,548	7,052,152
Debt Securities, Available-for-sale	9,019,788	7,059,674
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, within One Year, Amortized Cost	1,507	30,746
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, within One Year, Fair Value	1,507	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	655,609	406,325
Maturing in one through five years, cost	71,262	24,496
Maturing in five through ten years, cost	456,620	185,209
Maturing in over ten years, cost	14,965	109,872
Maturing in one year or less, fair value	655,779	406,927
Maturing in one through five years, fair value	72,446	24,971
Maturing in five through ten yeas, fair value	461,847	187,868
Maturing in over ten years, fair value	14,897	110,026
Agency Securities
Investment securities held to maturity
Investment securities available for sale at cost	659,260	684,085
Debt Securities, Available-for-sale	654,941	682,772
Residential Mortgage Backed Securities
Investment securities held to maturity
Investment securities held to maturity at cost	614,489	0
Commercial mortgage-backed securities	627,394	0
Investment securities held to maturity at fair value	627,394
Investment securities available for sale at cost	5,968,192	5,269,060
Debt Securities, Available-for-sale	6,069,668	5,267,090
Commercial Mortgage Backed Securities
Investment securities available for sale
Total investment securities available for sale, cost	1,058,640	373,105
Total investment securities available for sale, fair value	1,090,210	380,020
Investment securities held to maturity
Investment securities held to maturity at cost	130,987	0
Commercial mortgage-backed securities	131,602	0
Investment securities held to maturity at fair value	$ 131,602
Residential mortgage-backed securities
Investment securities held to maturity
Investment securities held to maturity at cost		30,996
Investment securities held to maturity at fair value		$ 30,996