Equity Compensation - Stock Options Outstanding by Exercise Price (Details) - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options Outstanding - Weighted Average Exercise Price (in dollars per share)	$ 19.44	$ 21.19	$ 21.22	$ 22.74
Stock Options
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Exercise price range, lower limit (in dollars per shares)	12.15	13.37	13.37
Exercise price range, upper limit (in dollars per shares)	$ 33.36	$ 33.36	$ 33.36
Options Outstanding - Number Outstanding at end of period (in shares)	2,631,569
Options Outstanding - Weighted Average Remaining Contractual Life	3 years 11 months
Options Outstanding - Weighted Average Exercise Price (in dollars per share)	$ 19.44
Options Exercisable - Number Exercisable at end of period (in shares)	2,029,773
Options Exercisable - Weighted Average Exercise Price (in dollars per share)	$ 21.60
Stock Options | $ 12.15 – $20.00
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Exercise price range, lower limit (in dollars per shares)	12.15
Exercise price range, upper limit (in dollars per shares)	$ 20
Options Outstanding - Number Outstanding at end of period (in shares)	1,071,784
Options Outstanding - Weighted Average Remaining Contractual Life	6 years 11 months
Options Outstanding - Weighted Average Exercise Price (in dollars per share)	$ 13.07
Options Exercisable - Number Exercisable at end of period (in shares)	469,988
Options Exercisable - Weighted Average Exercise Price (in dollars per share)	$ 14.23
Stock Options | $ 20.01 – $25.00
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Exercise price range, lower limit (in dollars per shares)	20.01
Exercise price range, upper limit (in dollars per shares)	$ 25
Options Outstanding - Number Outstanding at end of period (in shares)	823,501
Options Outstanding - Weighted Average Remaining Contractual Life	2 years 2 months
Options Outstanding - Weighted Average Exercise Price (in dollars per share)	$ 22.21
Options Exercisable - Number Exercisable at end of period (in shares)	823,501
Options Exercisable - Weighted Average Exercise Price (in dollars per share)	$ 22.21
Stock Options | $ 25.01 – $30.00
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Exercise price range, lower limit (in dollars per shares)	25.01
Exercise price range, upper limit (in dollars per shares)	$ 30
Options Outstanding - Number Outstanding at end of period (in shares)	731,788
Options Outstanding - Weighted Average Remaining Contractual Life	1 year 6 months
Options Outstanding - Weighted Average Exercise Price (in dollars per share)	$ 25.55
Options Exercisable - Number Exercisable at end of period (in shares)	731,788
Options Exercisable - Weighted Average Exercise Price (in dollars per share)	$ 25.55
Stock Options | $ 30.01 – $33.36
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Exercise price range, lower limit (in dollars per shares)	30.01
Exercise price range, upper limit (in dollars per shares)	$ 33.36
Options Outstanding - Number Outstanding at end of period (in shares)	4,496
Options Outstanding - Weighted Average Remaining Contractual Life	3 years 5 months
Options Outstanding - Weighted Average Exercise Price (in dollars per share)	$ 33.36
Options Exercisable - Number Exercisable at end of period (in shares)	4,496
Options Exercisable - Weighted Average Exercise Price (in dollars per share)	$ 33.36