Employee Stock and Incentive Plans - Employee Stock Ownership Plan (Details) - USD ($) shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Employee Stock Ownership Plan [Abstract]
ESOP Employer Loan	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 common stock. The note was refinanced in 2006 at 6 percent and subsequently matured in December 2015.
ESOP Compensation Expense	$ 9.2	$ 9.0	$ 9.1
ESOP Shares, Allocated	1.6	1.8	1.9
ESOP Shares, Unallocated	0.0	0.0	0.3
ESOP Shares, Total	1.6	1.8	2.2
ESOP Shares, Fair Value of Unallocated Shares	$ 0.0	$ 0.0	$ 13.2