Investment Securities - Maturities of Investment Securities (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis, Rolling Maturity
Available-for-sale amortized cost, within one year	$ 477.8
Available-for-sale amortized cost, after one year but within five years	1,519.2
Available-for-sale amortized cost, after five years but within ten years	154.5
Available-for-sale amortized cost, after ten years	153.7
Amortized Cost	2,305.2	$ 2,229.3
Available-for-sale Securities, Debt Maturities, Fair Value, Rolling Maturity
Available-for-sale estimated fair value, within one year	472.3
Available-for-sale estimated fair value, after one year but within five years	1,495.0
Available-for-sale estimated fair value, after ten years	151.9
Available-for-sale estimated fair value, after five years but within ten years	151.5
Available-for-Sale, Estimated Fair Value	2,270.7	2,208.7
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount
Held-to-maturity amortized cost, within one year	64.1
Held-to-maturity amortized cost, after one year but within five years	221.6
Held-to-maturity amortized cost, after five years but within ten years	94.4
Held-to-maturity amortized cost, after ten years	26.7
Amortized Cost	406.8	484.5
Held-to-maturity Securities, Debt Maturities, Fair Value, Rolling Maturity
Held-to-maturity estimated fair value, within one year	63.0
Held-to-maturity estimated fair value, after one year but within five years	216.7
Held-to-maturity estimated fair value, after five years but within ten years	94.9
Held-to-maturity estimated fair value, after ten years	26.1
Estimated Fair Value	400.7	483.3
Available-for-sale Securities and Held-to-maturity Securities
Securities sold under agreements to repurchase	1,943.1	2,087.7
Pledged assets separately reported, securities pledged for repurchase agreements, at fair value	1,908.4	2,062.6
Callable within one year
Available-for-sale Securities and Held-to-maturity Securities
Investment securities primarily classified as available-for-sale, amortized costs, after one year but within five years	2.0
Investment Securities,Callable Structured, Fair Value	2.0
Investment securities primarily classified as available-for-sale, fair value, after one year but within five years	64.7	64.8
Mortgage Backed Securities, Variable Rate
Available-for-sale Securities and Held-to-maturity Securities
Available-for-sale securities, amortized cost basis	$ 219.5	$ 247.9