Earnings Per Common Share - Antidilutive Shares Excluded from the Computation (Details) - shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive shares excluded from computations of diluted earnings per share (in shares)	596,061	829,469	1,256,249
Stock options, performance and restricted shares
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive shares excluded from computations of diluted earnings per share (in shares)	596,061	829,469	897,800
Assumed conversion of exchangeable note hedges
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive shares excluded from computations of diluted earnings per share (in shares)	0	0	358,449