Income Taxes - Summary of Tax Effects of Temporary Differences (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Schedule Of Deferred Tax Assets And Liabilities [Line Items]
Net operating loss carry forward		$ 0.4
Tax basis goodwill and intangibles	39.7
Depreciation and amortization	10.3	3.0
Deferred restricted common unit compensation	9.2
Accrued compensation	13.1	10.3
Other	9.1	4.3
Total deferred tax assets	81.4	18.0
Intangible assets	18.9	15.1
Unrealized appreciation on investments	54.4	33.0
Other	2.1	0.2
Total deferred tax liabilities	61.1	48.3
Net deferred tax assets (liabilities)	$ 6.0	$ (30.3)