Stockholders' Equity- Shares of Common Stock Issued During 2014 under Employee Stock Purchase Plan (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares Available, Beginning Balance			111,432
Shares Authorized for Issuance	250,000		250,000
Shares Issued during Period (Shares)	36,305.0	38,459.0	50,185
Shares Available, Ending Balance	236,483
Shares Issued during Period (per Share Price)	$ 21.49	$ 20.39	$ 15.58