Stock-Based Compensation (Details 3) (USD $) In Millions, except Share data in Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Other Disclosures
Total unrecognized compensation cost related to the unvested awards	$ 20.2
Total weighted average remaining term of unvested awards	1 year 10 months 24 days
Compensation cost recognized	8.6	9.6	6.6
Performance Shares
Summary of unvested restricted stock and deferred shares activity and related information
Granted (in shares)	117
Cancelled/Forfeitures (in shares)	(10)
Unvested at end of year (in shares)	107
Weighted Average Grant Date Fair Value
Granted (in dollars per share)	$ 57.02
Cancelled/Forfeitures (in dollars per share)	$ 57.47
Unvested at end of period (in dollars per share)	$ 56.97
Other Disclosures
Total unrecognized compensation cost related to the unvested awards	5.0
Total weighted average remaining term of unvested awards	2 years
Compensation cost recognized	0.7
Aggregate intrinsic value of restricted stock granted and outstanding	0.7
Performance Shares | Class A
Other Disclosures
Company's closing Common Stock price (in dollars per share)	$ 63.44
Second Amended and Restated 2004 Stock Incentive Plan | Restricted stock and deferred shares
Summary of unvested restricted stock and deferred shares activity and related information
Unvested at beginning of year (in shares)	260	237	153
Granted (in shares)	151	142	170
Cancelled/Forfeitures (in shares)	(95)	(16)	(8)
Vested (in shares)	(102)	(103)	(78)
Unvested at end of year (in shares)	214	260	237
Weighted Average Grant Date Fair Value
Unvested at beginning of period (in dollars per share)	$ 45.58	$ 35.45	$ 30.33
Granted (in dollars per share)	$ 56.79	$ 54.80	$ 37.62
Cancelled/Forfeitures (in dollars per share)	$ 46.83	$ 37.44	$ 30.66
Vested (in dollars per share)	$ 44.87	$ 35.25	$ 30.61
Unvested at end of period (in dollars per share)	$ 53.74	$ 45.58	$ 35.45
Other Disclosures
Total fair value of shares vested	5.9	5.6	2.5
Total unrecognized compensation cost related to the unvested awards	9.0
Total weighted average remaining term of unvested awards	1 year 8 months 12 days
Compensation cost recognized	4.8	5.1	3.1
Aggregate intrinsic value of restricted stock granted and outstanding	12.2
Second Amended and Restated 2004 Stock Incentive Plan | Restricted stock and deferred shares | Class A
Other Disclosures
Company's closing Common Stock price (in dollars per share)	$ 63.44
Management Stock Purchase Plan | Restricted stock | Class A
Other Disclosures
Company's closing Common Stock price (in dollars per share)	$ 63.44
Management Stock Purchase Plan | Restricted stock units (RSUs)
Summary of unvested restricted stock and deferred shares activity and related information
Granted (in shares)	31	45	64
Cancelled/Forfeitures (in shares)	(32)	(14)	(110)
Weighted Average Grant Date Fair Value
Granted (in dollars per share)	$ 22.57	$ 18.05	$ 15.68
Other Disclosures
Total unrecognized compensation cost related to the unvested awards	0.6
Total weighted average remaining term of unvested awards	1 year 4 months 24 days
Compensation cost recognized	0.5	0.7	0.8
Dividend declared and unpaid	$ 0.1