Investment Securities - Maturities of Investment Securities (Details) - USD ($) $ in Millions	Dec. 31, 2019	Jan. 01, 2019	Dec. 31, 2018
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis, Rolling Maturity
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling within One Year, Amortized Cost	$ 677.3
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after One Through Five Years, Amortized Cost	1,486.2
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after Five Through Ten Years, Amortized Cost	328.0
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after 10 Years, Amortized Cost	454.0
Amortized Cost	2,945.5		$ 2,305.2
Available-for-sale Securities, Debt Maturities, Fair Value, Rolling Maturity
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling within One Year, Fair Value	661.4
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after One Through Five Years, Fair Value	1,470.9
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after Five Through Ten Years, Fair Value	428.2
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after 10 Years, Fair Value	399.5
Estimated Fair Value	2,960.0		2,270.7
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, within One Year, Amortized Cost	38.6
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Amortized Cost	29.7
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after Five Through Ten Years, Amortized Cost	21.3
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after 10 Years, Amortized Cost	2.7
Amortized Cost	92.3	$ 281.1	406.8
Held-to-maturity Securities, Debt Maturities, Fair Value, Rolling Maturity
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Rolling within One Year, Fair Value	38.6
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Rolling after One Through Five Years, Fair Value	30.1
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Rolling after Five Through Ten Years, Fair Value	23.1
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Rolling after 10 Years, Fair Value	2.7
Estimated Fair Value	94.5	$ 275.1	400.7
Available-for-sale Securities and Held-to-maturity Securities
Securities sold under agreements to repurchase	2,132.0		1,943.1
Pledged assets separately reported, securities pledged for repurchase agreements, at fair value	2,144.9		1,908.4
Mortgage Backed Securities, Variable Rate
Available-for-sale Securities and Held-to-maturity Securities
Available-for-sale securities, amortized cost basis	$ 298.1		$ 219.5