Mortgage Notes Payable (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Debt Instrument [Line Items]
Mortgage notes payable	$ 2,245,944,000	$ 1,404,080,000
Number of properties securing mortgage notes	67	49
Weighted average interest rate (in hundredths)	4.60%	5.60%
Monthly interest expense	9,400,000
Monthly principal amortization	3,300,000
Mortgages notes payable	2,245,944,000	1,404,080,000
Repayment of debt before the scheduled maturity date, prepayment penalties	The prepayment penalty on the majority of the Company’s mortgage notes payable are computed by the greater of (a) 1% of the amount of the principal being prepaid or (b) the present value of the mortgage note payable which is calculated by multiplying the principal being prepaid by the difference between the interest rate of the mortgage note and the stated yield rate on a specified U.S. treasury security as defined in the mortgage note agreement.
Minimum [Member]
Debt Instrument [Line Items]
Remaining terms	1 year	1 year
Maximum [Member]
Debt Instrument [Line Items]
Remaining terms	26 years	26 years
Secured Debt [Member]
Debt Instrument [Line Items]
Mortgage notes payable	2,162,151,000
Debt Instrument, Unamortized Premium	83,800,000	11,500,000
Mortgage Notes Payable [Member]
Debt Instrument [Line Items]
Mortgage notes payable	2,056,742,000	1,236,479,000
Prepayment Penalty, Percent Of Principal Prepaid	1.00%
Multifamily Housing Mortgage Revenue Bonds [Member]
Debt Instrument [Line Items]
Mortgage notes payable	189,202,000	167,601,000
Multifamily housing mortgage revenue bonds, variable interest rate (in hundredths)		1.80%	1.60%
Multifamily housing mortgage revenue bonds credit enhancement and underwriting fees, minimum (in hundredths)		1.20%	1.20%
Multifamily housing mortgage revenue bonds credit enhancement and underwriting fees, maximum (in hundredths)		1.90%	1.90%
Percentage of units subject to tenant income criteria (in hundredths)	20.00%
Bond subject to interest rate caps	153,200,000
Secured Deeds of Trust [Member]
Debt Instrument [Line Items]
Mortgages notes payable	$ 58,200,000