Equity (Details) (USD $) In Thousands, except Share data, unless otherwise specified	9 Months Ended			3 Months Ended			9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Mar. 01, 2013	Sep. 30, 2013 At The Market Stock Sales [Member]	Jun. 30, 2013 At The Market Stock Sales [Member]	Mar. 31, 2013 At The Market Stock Sales [Member]	Sep. 30, 2013 At The Market Stock Sales [Member]
Targeted or Tracking Stock, Stock [Line Items]
Common Stock Offering, Maximum Aggregate Price			$ 300,000
number of shares sold				0	6,530,193	1,889,105	8,419,298
Gross Proceeds from sale of common stock				0	167,034	44,459	211,493
Proceeds from issuances of common stock	209,510	128		0	165,692	43,904	209,596
proceeds from sale of common stock weighted average price per share				$ 0.00	$ 25.58	$ 23.53	$ 25.12
Commission To Sales Agent, Percent			2.00%
Stock Issued During Period, Shares, New Issues	8,419,298
Common Stock Offering Maximum Aggregate Price Still Available	$ 88,507