Equity - Share-Based Compensation Narrative (Details)	12 Months Ended
	Dec. 31, 2017 USD ($) $ / shares shares	Dec. 31, 2016 USD ($) $ / shares shares	Dec. 31, 2015 USD ($) $ / shares shares
Class of Stock [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested and Expected to Vest, Outstanding, Number | shares	163,272
Proceeds from Stock Options Exercised	$ 200,000	$ 500,000	$ 1,700,000
Employee Service Share-based Compensation, Tax Benefit from Exercise of Stock Options	2,600,000	3,600,000	800,000
Adjustments to Additional Paid in Capital, Share-based Compensation and Exercise of Stock Options	$ (2,600,000)	$ (3,100,000)	$ (600,000)
Stock Issued During Period, Shares, Other | shares	698	1,130	491
Stock Issued During Period, Value, Other	$ 41,000	$ 41,000	$ 14,000
Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested and Expected to Vest, Exercisable, Weighted Average Exercise Price | $ / shares	$ 32.95
Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested and Expected to Vest, Outstanding, Aggregate Intrinsic Value	$ 14,800,000	17,400,000
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period, Intrinsic Value	3,800,000	$ 7,600,000	$ 900,000
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized	$ 2,300,000
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition	1 year 8 months 5 days
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Weighted Average Grant Date Fair Value | $ / shares	$ 44.93	$ 25.11	$ 17.97
Allocated Share-based Compensation Expense	$ 2,700,000	$ 3,300,000	$ 3,300,000
Common Class C, Non-Voting [Member]
Class of Stock [Line Items]
Number of Board of Directors	4	4	4