Stock Purchase Plan Stock Purchase Plan (Details) (USD $)	3 Months Ended			12 Months Ended
	Dec. 31, 2014	Sep. 30, 2013	Jun. 30, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
stock offering			11,100,000
Proceeds from Issuance of Common Stock		$ 20,000,000	$ 47,000,000	$ 0	$ 67,000,000	$ 0
Share Price		$ 29.50
Common Stock, Discount on Shares		0.96
Equity Forward Sale Agreement, Settlement Threshold	10,400,000
equity forward sale agreement future settlement amount	275,000,000
Option Indexed to Issuer's Equity, Settlement Alternatives, Cash, at Fair Value	119,000,000			119,000,000
Net share settlement	3,135,000			3,135,000
employee stock purchase plan shares authorized	625,000			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	427,021			427,021
Dividend Reinvestment and Direct Stock Purchase Plan Shares				2,500,000
Dividend Reinvestment and Direct Stock Purchase Plan, Number of Available Shares	2,481,110			2,481,110
Common Stock [Member]
Stock Issued During Period, Shares, New Issues		700,000	1,665,000		2,365,000