Income Per Share of Common Stock - Potentially Dilutive Securities Excluded from Computation (Details) - Convertible Debt Securities - 4 1/16% Convertible Subordinated Debentures - shares shares in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total potentially dilutive securities	0.0	0.3
Debt instrument interest rate stated percentage	4.0625%