Debt - Narrative (Detail) (USD $)	3 Months Ended		1 Months Ended	3 Months Ended			3 Months Ended			3 Months Ended
	Mar. 31, 2014	Dec. 31, 2013	Dec. 20, 2010 AMH Credit Agreement	Mar. 31, 2014 AMH Credit Agreement	Mar. 31, 2013 AMH Credit Agreement	Apr. 20, 2007 AMH Credit Agreement	Mar. 31, 2014 2013 AMH Credit Agreement	Dec. 31, 2013 2013 AMH Credit Agreement	Mar. 31, 2014 Term Loan held by affiliate	Mar. 31, 2014 Revolving Credit Facility	Mar. 31, 2014 Minimum	Mar. 31, 2014 Maximum	Mar. 31, 2014 Minimum	Mar. 31, 2014 Maximum	Mar. 31, 2014 Minimum	Mar. 31, 2014 Maximum	Mar. 31, 2014 2013 AMH Credit Agreement
Debt Instrument [Line Items]
Interest payable under credit agreement	$ 750,000,000	$ 750,000,000				$ 1,000,000,000	$ 750,000,000	$ 750,000,000	$ 271,700,000	$ 500,000,000
Interest payable under credit agreement period				7 years
Estimated fair value of long debt obligation			995,000,000	751,900,000
Fair value debt repurchased			90,900,000	995,000,000
Repurchase of term loan			180,800,000
AMH credit agreement remaining balance			728,300,000
Debt Instrument, Basis Spread on Variable Rate											1.125%	1.75%			0.125%	0.75%
Interest rate face amount				723,300,000
Debt Instrument, Maturity Date, Description				Three-month LIBOR
Interest on AMH credit agreement remaining balance				4.00%			1.36%
Portion of Loan				5,000,000
Interest rate on remaining portion of loan				1.25%
Interest expense incurred				7,300,000			2,700,000
Amortization of Debt					100,000		300,000
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage													0.125%	0.25%
Debt Issuance Cost	6,600,000
Assets under Management, Carrying Amount	$ 40,000,000,000
Leverage Ratio	4.00									3.75							1.00
Euro Dollar Rate Plus Under Federal Fund Rate				One-month Eurodollar Rate plus