Share-based Compensation Plans (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation cost	$ 14.9	$ 11.4	$ 11.9
Income tax benefit from compensation cost	$ 5.2	$ 4.0	$ 4.2
Shares [Roll Forward]
Restricted stock outstanding at end of period (in shares)	3,300,000
Restricted Stock/Restricted Stock Units
Weighted average grant date fair market value [Abstract]
Restricted stock outstanding at end of period (in dollars per share)	$ 7.44
Granted (in dollars per share)	10.41	$ 5.66	$ 9.03
Vested (in dollars per share)	7.71
Forfeited (in dollars per share)	8.43
Restricted stock outstanding at end of period (in dollars per share)	$ 8.78	$ 7.44
Shares [Roll Forward]
Restricted stock outstanding at beginning of period (in shares)	3,146,672
Granted (in shares)	1,631,744
Vested (in shares)	(1,409,347)
Forfeited (in shares)	(68,460)
Restricted stock outstanding at end of period (in shares)	3,300,609	3,146,672
Additional disclosures [Abstract]
Restricted stock, performance shares (in shares)	2,500,000
Restricted stock, time vested shares (in shares)	800,000
Total fair value of restricted stock vested	$ 15.3	$ 12.2	$ 17.2
Unrecognized compensation cost	13.2
Unrecognized compensation cost, performance shares	9.9
Unrecognized compensation cost, time vested shares	$ 3.3
Weighted-average period for recognition of compensation cost	1 year 6 months
2015 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum number of shares can be awarded under the plan (in shares)	10,000,000
Additional disclosures [Abstract]
Shares available for future grants (in shares)	6,700,000
Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period	1 year
Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period	3 years