Common Stock (Details) (USD $)	0 Months Ended	9 Months Ended					0 Months Ended				1 Months Ended		0 Months Ended
	Sep. 26, 2014	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014 At-the-Market	May 28, 2014 At-the-Market	May 28, 2014 Maximum At-the-Market	Oct. 02, 2014 Subsequent Event	Sep. 26, 2014 Common Stock	Feb. 11, 2014 Common Stock	Jan. 10, 2014 Common Stock	Feb. 11, 2014 Common Stock	Sep. 26, 2014 Common Stock	Oct. 02, 2014 Common Stock Subsequent Event	Oct. 02, 2014 Common Stock Subsequent Event
Class of Stock [Line Items]
Issuance of shares of common stock (in shares)		22,216,900		391,900				10,000,000	1,325,000	10,500,000			1,500,000
Stock offering price (dollars per share)									$ 18.15	$ 18.15	$ 18.15	$ 18.60		$ 18.60
Proceeds from stock offerings, net of offering costs	$ 178,000,000	$ 391,134,000	$ 209,768,000	$ 8,000,000			$ 26,800,000				$ 205,200,000
Amount of gross proceeds authorized					150,000,000
Percentage of sales commissions on stock sale price						2.00%
Sales commissions				$ 122,000