Share-Based Compensation - Nonvested share activity and related fair value (Details) - Nonvested stock awards - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Nonvested Stock Shares
Nonvested at the beginning of the period (in shares)	3,405,207
Granted (in shares)	2,700,977
Vested (in shares)	(1,207,677)
Forfeited (in shares)	(112,404)
Nonvested at the end of the period (in shares)	4,786,103	3,405,207
Nonvested Stock Shares, Weighted Average Grant Date Fair Value
Nonvested at the beginning of the period (in dollars per share)	$ 50.09
Granted (in dollars per share)	21.89
Vested (in dollars per share)	48.70
Forfeited (in dollars per share)	40.96
Nonvested at the end of the period (in dollars per share)	$ 34.74	$ 50.09
Compensation expense	$ 51.5	$ 47.5	$ 54.1
Related income tax benefit recognized	19.2	17.6	20.1
Remaining unamortized expense of nonvested stock expected to be recognized	$ 112.5
Period for expense amortized	2 years 8 months 12 days
Total fair value of shares vested	$ 26.7	$ 53.9	$ 104.8