Preferred Stock (Details) - USD ($) $ / shares in Units, $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2015	Dec. 31, 2014
Class of Stock [Line Items]
Shares Authorized	5,959,100	5,959,100
Shares Issued and Outstanding	102,000	102,000
Liquidation Preference	$ 975,000	$ 975,000
Par Value	$ 1.00	$ 1.00
Preferred Stock [Member]
Class of Stock [Line Items]
Shares Authorized	105,450	105,450
Shares Issued and Outstanding	102,000	102,000
Liquidation Preference	$ 975,000	$ 975,000
Series H Preferred Stock [Member]
Class of Stock [Line Items]
Shares Authorized	70,000	70,000
Shares Issued and Outstanding	70,000	70,000
Liquidation Preference	$ 175,000	$ 175,000
Dividend Rate	6.90%	6.90%
Annual Dividend per Depositary Share	$ 1.72500	$ 1.72500
Par Value	$ 1.00	$ 1.00
Series I Preferred Stock [Member]
Class of Stock [Line Items]
Shares Authorized	18,400	18,400
Shares Issued and Outstanding	16,000	16,000
Liquidation Preference	$ 400,000	$ 400,000
Dividend Rate	6.00%	6.00%
Annual Dividend per Depositary Share	$ 1.50000	$ 1.50000
Par Value	$ 1.00	$ 1.00
Series J Preferred Stock [Member]
Class of Stock [Line Items]
Shares Authorized	9,000	9,000
Shares Issued and Outstanding	9,000	9,000
Liquidation Preference	$ 225,000	$ 225,000
Dividend Rate	5.50%	5.50%
Annual Dividend per Depositary Share	$ 1.37500	$ 1.37500
Par Value	$ 1.00	$ 1.00
Series K Preferred Stock [Member]
Class of Stock [Line Items]
Shares Authorized	8,050	8,050
Shares Issued and Outstanding	7,000	7,000
Liquidation Preference	$ 175,000	$ 175,000
Dividend Rate	5.625%	5.625%
Annual Dividend per Depositary Share	$ 1.40625	$ 1.40625
Par Value	$ 1.00	$ 1.00