Employee Stock and Incentive Plans - Employee Stock Ownership Plan (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Employee Stock and Incentive Plans [Abstract]
ESOP Employer Loan, Description	In 1990, the ESOP issued a $75.0 million note (term not to exceed 25 years at 10.25 percent) to use as consideration for 2.8 million shares (1.9 million shares adjusted for stock splits) of our newly issued common stock. The note was refinanced in 2006 at 6 percent.
ESOP Compensation Expense	$ 9.1	$ 8.4	$ 7.7
ESOP Shares, Allocated	1.9	2.0	2.2
ESOP Shares, Unallocated	0.3	0.5	0.7
ESOP Shares, Total	2.2	2.5	2.9
Fair Value of Unallocated ESOP Shares	$ 13.2	$ 24.1	$ 28.7