Aggregate dividends declared during the period for each share of common stock outstanding.

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Equity (Narratives) (Details) (USD $)	0 Months Ended	3 Months Ended								12 Months Ended			1 Months Ended
Equity (Narratives) (Details) (USD $)	Oct. 19, 2012	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Oct. 31, 2014	Oct. 01, 2013	Dec. 31, 2011
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Per shares distributions declared		$ 0.950	$ 0.940	$ 0.900	$ 0.895	$ 0.980	$ 0.860	$ 0.840	$ 0.820	$ 3.685	$ 3.390	$ 2.440
Redeemable Noncontrolling Interest
Redeemable noncontrolling interest		$ 6,071,000				$ 7,436,000				$ 6,071,000	$ 7,436,000
Common stock, par value		$ 0.001	$ 0.001			$ 0.001				$ 0.001	$ 0.001
Share price			$ 64.00us-gaap_SharePrice
Proceeds from issuance of shares in public offering										282,162,000	0	0
Repayments of debt											48,700,000us-gaap_RepaymentsOfDebt	21,200,000us-gaap_RepaymentsOfDebt
Sales of Common Stock
Shares issued in public offering			4,600,000us-gaap_SharesIssued
Common stock, issued	937,500us-gaap_CommonStockSharesIssued	105,085,069us-gaap_CommonStockSharesIssued				69,299,949us-gaap_CommonStockSharesIssued				105,085,069us-gaap_CommonStockSharesIssued	69,299,949us-gaap_CommonStockSharesIssued
Sales price of stock (per share)	$ 48us-gaap_SaleOfStockPricePerShare
Proceeds from issuance of common stock	45,000,000
WPCI
Redeemable Noncontrolling Interest
Minority interest ownership														7.70%
Underwriters option
Sales of Common Stock
Shares issued in public offering			600,000
Additional Paid-in Capital
Redeemable Noncontrolling Interest
Proceeds from issuance of shares in public offering										282,157,000
Redeemable noncontrolling interest
Redeemable Noncontrolling Interest
Redeemable noncontrolling interest		6,071,000				7,436,000				6,071,000	7,436,000	7,531,000			7,700,000
Senior Unsecured Credit Facility
Redeemable Noncontrolling Interest
Repayments of debt													$ 225,800,000