Equity, Stock Repurchase Program, Dividends, and Shelf Registration (Details) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended	11 Months Ended	12 Months Ended			203 Months Ended	204 Months Ended
	Dec. 31, 2015	Nov. 30, 2015	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Nov. 30, 2015	Dec. 31, 2015	Aug. 31, 2015	Nov. 06, 2013	Feb. 05, 2013	Dec. 31, 1999
Dividends Payable [Line Items]
Dividends paid (in dollars per share)			$ 0.28	$ 0.50	$ 0.72
Dividend cost			$ 7.5	$ 12.9	$ 18.7
Share based compensation dividends accrued	$ 0.6		$ 0.6	0.6			$ 0.6
Debt Instrument [Line Items]
Amount available for issuance under shelf registration agreement				$ 500.0
GBL [Member]
Stock Repurchase Program [Abstract]
Closing prices on shares (in dollars per share)	$ 31.04		$ 31.04				$ 31.04
AC [Member]
Stock Repurchase Program [Abstract]
Closing prices on shares (in dollars per share)	$ 30.50		$ 30.50				$ 30.50
Common Class A [Member] | Stock Repurchase Program [Member]
Stock Repurchase Program [Abstract]
Incremental Class A shares authorized to buyback											$ 9.0
Incremental Class A shares authorized to buyback (in shares)								500,000	500,000	500,000
Shares repurchased (in shares)	13,400	413,228	426,628	414,432	229,228	9,539,253	9,552,653
Average price per share of repurchased shares (in dollars per share)	$ 32.56	$ 64.86	$ 63.85	$ 78.99	$ 64.41		$ 44.81
Share available under program to repurchase (in shares)	582,155		582,155				582,155
Cost of shares repurchased							$ 428.0