Stockholders' Equity - Common Stock (Details) (USD $)	0 Months Ended	3 Months Ended		0 Months Ended	1 Months Ended															12 Months Ended
	May 04, 2015	Mar. 31, 2015	Mar. 31, 2014	Feb. 20, 2015	Mar. 31, 2015	Feb. 28, 2015	Jan. 31, 2015	Dec. 31, 2014	Nov. 30, 2014	Oct. 31, 2014	Sep. 30, 2014	Aug. 31, 2014	Jul. 31, 2014	Jun. 30, 2014	May 31, 2014	Apr. 30, 2014	Mar. 31, 2014	Feb. 28, 2014	Jan. 31, 2014	Dec. 31, 2014
Stockholders' Equity
Net Proceeds		$ 10,133,000	$ 77,568,000
Dividends declared (in dollars per share)	$ 0.1150
Common Stock
Stockholders' Equity
Dividends declared (in dollars per share)		$ 0.3375		$ 0.1125	$ 0.1125	$ 0.1125	$ 0.1125	$ 0.110	$ 0.110	$ 0.110	$ 0.110	$ 0.110	$ 0.110	$ 0.105	$ 0.105	$ 0.105	$ 0.105	$ 0.105	$ 0.105	$ 1.2900
Common Stock | 2012 $75 million ATM
Stockholders' Equity
Shares sold																				661,930
Average Price per Share (in dollars per share)								$ 22.47												$ 22.47
Gross Proceeds																				14,900,000
Sales Agents' Fees																				200,000
Net Proceeds																				14,700,000
Common Stock | 2014 $150 million ATM
Stockholders' Equity
Maximum Aggregate Offering Price		150,000,000			150,000,000
Aggregate Common Stock Available		17,321,000			17,321,000
Shares sold																				5,760,651
Average Price per Share (in dollars per share)								$ 23.03												$ 23.03
Gross Proceeds																				132,700,000
Sales Agents' Fees																				2,000,000
Net Proceeds																				130,700,000
Common Stock | 2014 $200 million ATM
Stockholders' Equity
Maximum Aggregate Offering Price		200,000,000			200,000,000
Aggregate Common Stock Available		154,818,000			154,818,000
Shares sold		417,115																		1,658,795
Average Price per Share (in dollars per share)		$ 24.29			$ 24.29			$ 21.13												$ 21.13
Gross Proceeds		10,100,000																		35,000,000
Sales Agents' Fees		100,000																		500,000
Net Proceeds		10,000,000																		34,500,000
Common Stock | At The Market (ATM) Program
Stockholders' Equity
Shares sold		417,115																		8,081,376
Average Price per Share (in dollars per share)		$ 24.29			$ 24.29			$ 22.60												$ 22.60
Gross Proceeds		10,100,000																		182,600,000
Sales Agents' Fees		100,000																		2,700,000
Net Proceeds		$ 10,000,000																		$ 179,900,000