Stockholders' Equity - At the Market Equity Offering Program (Details) (USD $) In Thousands, except Share data, unless otherwise specified	0 Months Ended		12 Months Ended			1 Months Ended
	Jul. 19, 2013	Sep. 28, 2012	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Jan. 31, 2015
Class of Stock [Line Items]
Issuance of common stock	3,000,000	9,200,000
Net proceeds from issuance of common stock	$ 78,300	$ 201,100	$ 76,856	$ 220,252	$ 202,352
At The Market Equity Offering Program [Member]
Class of Stock [Line Items]
Issuance of common stock			3,009,761	5,207,871
Net proceeds from issuance of common stock			75,700	140,600
Number of authorized shares remaining under offering program			2,381,639
Minimum [Member] | At The Market Equity Offering Program [Member]
Class of Stock [Line Items]
Issuance of common stock (price per share)			$ 24.35	$ 24.19
Maximum [Member] | At The Market Equity Offering Program [Member]
Class of Stock [Line Items]
Issuance of common stock (price per share)			$ 27.53	$ 30.49
Subsequent Event [Member] | At The Market Equity Offering Program [Member]
Class of Stock [Line Items]
Issuance of common stock						531,153
Net proceeds from issuance of common stock						$ 14,900