Changes in Capital Structure - Narrative (Details) - USD ($) $ in Millions			3 Months Ended
	Jun. 13, 2016	May 24, 2016	Jun. 30, 2016	Sep. 30, 2017	Apr. 27, 2017	Dec. 31, 2016
Capital Structure
Common stock, shares authorized (in shares)				500,000,000		500,000,000
Redemption of shares (as percent)			100.00%
Increase in shares available for issuance under the ESPP					3,000,000
Shares of treasury stock available for issuance under the ESPP				3,107,050
Increase in shares available for Issuance,under Amended and Restated Long-term Incentive Plan					3,000,000
Convertible Preferred Stock
Capital Structure
Redemption of shares (as percent)	100.00%	100.00%
Preferred shares repurchased (in shares)		$ 344.5
Preferred stock, dividend rate amended (as percent)		2.822%	2.822%
Repurchase of outstanding shares	$ 226.0
Gain on redemption	78.0
Carrying value of preferred stock at time of redemption	$ 304.0