Stock Purchase Plan Stock Purchase Plan (Details) (USD $)	3 Months Ended		6 Months Ended	12 Months Ended
	Sep. 30, 2013	Jun. 30, 2013	Jun. 30, 2013	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Mar. 05, 2013
stock offering		11,100,000
Share Price	$ 29.50						$ 30.29
Equity Forward Sale Agreement, Settlement Threshold	10,400,000
equity forward sale agreement future settlement amount	$ 288,000,000
Option Indexed to Issuer's Equity, Settlement Alternatives, Cash, at Fair Value				26,000,000
Net share settlement				876,318
Common Stock, Discount on Shares	0.96
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	0
employee stock purchase plan shares authorized				625,000
Employee Stock Purchase Plan, Base Pay Threshold				10.00%
Employee Stock Purchase Plan, Purchased Stock Value Limitation				$ 25,000
Employee Stock Purchase Plan, Share Purchase Limitation				1,500
Employee Stock Purchase Plan, Purchase Price				95.00%
Employee Stock Purchase Plan, Number of Available Shares				451,506
Dividend Reinvestment and Direct Stock Purchase Plan Shares				2,500,000
Dividend Reinvestment and Direct Stock Purchase Plan, Number of Available Shares				2,485,055
Common Stock [Member]
Stock Issued During Period, Shares, New Issues	700,000	1,665,000		2,365,000