Equity (Details) (USD $)	3 Months Ended	6 Months Ended	9 Months Ended
	Sep. 30, 2013	Jun. 30, 2013	Sep. 30, 2013	Sep. 30, 2012
Statement of Equity [Abstract]
stock offering	11,100,000
Sale of Stock, Price Per Share	$ 28.54		$ 28.54
Proceeds from Issuance of Common Stock	$ 20,000,000	$ 47,000,000	$ 67,000,000	$ 0
Share Price	$ 29.50		$ 29.50
Common Stock, Discount on Shares	0.96		0.96
Equity Forward Sale Agreement, Settlement Threshold	10,400,000
equity forward sale agreement future settlement amount	291,000,000
Option Indexed to Issuer's Equity, Settlement Alternatives, Cash, at Fair Value	$ 3,000,000		$ 3,000,000
Net share settlement	100,037		100,037