Equity (Details) (USD $)	3 Months Ended	12 Months Ended			122 Months Ended
	Mar. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012
Treasury Stock, Shares, Acquired	128,710				34,704,191
Treasury Stock, Value, Acquired, Cost Method	$ 15,800,000				$ 963,800,000
Treasury Stock Acquired, Average Cost Per Share					$ 27.77
Treasury Stock, Shares, Retired		1,138,224			28,675,824
Treasury Stock, Retired, Cost Method, Amount		0	0	0	552,800,000
Treasury Stock, Shares		1,550,173	1,408,334		1,550,173
Treasury shares, Remaining shares authorized to be purchased		3,239,785			3,239,785
Common Stock, Dividends, Per Share, Cash Paid		$ 0.28
Dividends		$ (52,950,000)	$ (46,027,000)	$ (39,791,000)
Common Stock, Dividends, Per Share, Declared	$ 0.32