Investment Securities (Schedule of AFS and HTM - Estimated Unrealized Losses) (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Total securities available-for-sale, Less than 12 months, Fair Value	$ 3,075.6	$ 2,525.8
Total securities available-for-sale, Less than 12 months, Gross Unrealized Loss	(36.0)	(38.4)
Total securities available-for-sale, 12 months or greater, Fair Value	200.4	29.6
Total securities available-for-sale, 12 months or greater, Gross Unrealized Loss	(3.9)	(0.9)
Schedule of Held-to-maturity Securities [Line Items]
Total securities held-to-maturity, Less than 12 months, Fair Value	48.4	72.0
Total securities held-to-maturity, Less than 12 months, Gross Unrealized Loss	(1.0)	(1.8)
Total securities held-to-maturity, 12 months or greater, Fair Value	42.4	49.1
Total securities held-to-maturity, 12 months or greater, Gross Unrealized Loss	(1.7)	(2.0)
Total, Less than 12 months, Fair Value	3,124.0	2,597.8
Total, Less than 12 months, Gross Unrealized Loss	(37.0)	(40.2)
Total, 12 months or greater, Fair Value	242.8	78.7
Total, 12 months or greater, Gross Unrealized Loss	(5.6)	(2.9)
U.S. government agency securities
Schedule of Available-for-sale Securities [Line Items]
Total securities available-for-sale, Less than 12 months, Fair Value	2,524.5	1,589.6
Total securities available-for-sale, Less than 12 months, Gross Unrealized Loss	(32.1)	(31.8)
Total securities available-for-sale, 12 months or greater, Fair Value	31.6	13.8
Total securities available-for-sale, 12 months or greater, Gross Unrealized Loss	(1.1)	(0.5)
Non-agency securities
Schedule of Available-for-sale Securities [Line Items]
Total securities available-for-sale, Less than 12 months, Fair Value	17.6	56.5
Total securities available-for-sale, Less than 12 months, Gross Unrealized Loss	(0.7)	(1.4)
Total securities available-for-sale, 12 months or greater, Fair Value	0.5	15.8
Total securities available-for-sale, 12 months or greater, Gross Unrealized Loss	(0.5)	(0.4)
U.S. government agency obligations
Schedule of Available-for-sale Securities [Line Items]
Total securities available-for-sale, Less than 12 months, Fair Value	397.3	546.1
Total securities available-for-sale, Less than 12 months, Gross Unrealized Loss	(2.6)	(3.9)
Total securities available-for-sale, 12 months or greater, Fair Value	148.2	0.0
Total securities available-for-sale, 12 months or greater, Gross Unrealized Loss	(1.8)	0.0
U.S. Treasury Securities
Schedule of Available-for-sale Securities [Line Items]
Total securities available-for-sale, Less than 12 months, Fair Value	121.8	299.5
Total securities available-for-sale, Less than 12 months, Gross Unrealized Loss	(0.2)	(0.4)
Total securities available-for-sale, 12 months or greater, Fair Value	0.0	0.0
Total securities available-for-sale, 12 months or greater, Gross Unrealized Loss	0.0	0.0
Debt securities
Schedule of Available-for-sale Securities [Line Items]
Total securities available-for-sale, Less than 12 months, Fair Value	3,061.2	2,491.7
Total securities available-for-sale, Less than 12 months, Gross Unrealized Loss	(35.6)	(37.5)
Total securities available-for-sale, 12 months or greater, Fair Value	180.3	29.6
Total securities available-for-sale, 12 months or greater, Gross Unrealized Loss	(3.4)	(0.9)
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Total securities available-for-sale, Less than 12 months, Fair Value	14.4	34.1
Total securities available-for-sale, Less than 12 months, Gross Unrealized Loss	(0.4)	(0.9)
Total securities available-for-sale, 12 months or greater, Fair Value	20.1	0.0
Total securities available-for-sale, 12 months or greater, Gross Unrealized Loss	(0.5)	0.0
U.S. government agency securities
Schedule of Held-to-maturity Securities [Line Items]
Total securities held-to-maturity, Less than 12 months, Fair Value	47.6	68.2
Total securities held-to-maturity, Less than 12 months, Gross Unrealized Loss	(1.0)	(1.7)
Total securities held-to-maturity, 12 months or greater, Fair Value	29.5	26.7
Total securities held-to-maturity, 12 months or greater, Gross Unrealized Loss	(1.4)	(1.6)
State and municipal
Schedule of Held-to-maturity Securities [Line Items]
Total securities held-to-maturity, Less than 12 months, Fair Value	0.8	3.8
Total securities held-to-maturity, Less than 12 months, Gross Unrealized Loss	0.0	(0.1)
Total securities held-to-maturity, 12 months or greater, Fair Value	12.9	22.4
Total securities held-to-maturity, 12 months or greater, Gross Unrealized Loss	$ (0.3)	$ (0.4)