Shareholders' Equity (Details) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Authorized Shares [Line Items]
Total authorized shares									200,000,000	200,000,000
Par value per share									$ 1	$ 1
Preferred Shares [Abstract]
Number of preferred shares issued	34,150,000				35,750,000				34,150,000	35,750,000
Total consideration									$ 241,265,000	$ 0	$ 361,722,000
Aggregate liquidation value	853,750,000				893,750,000				853,750,000	893,750,000
Loss on redemption of preferred shares	0	0	0	9,000,000	0	0	0	0	9,135,000	0	0
Common Shares [Member]
Authorized Shares [Line Items]
Total authorized shares									130,000,000	130,000,000
Common Shares [Abstract]
Number of shares repurchased									7,700,000	7,100,000	5,400,000
Total cost of shares repurchased									695,300,000	532,900,000	396,200,000
Average cost per share of shares repurchased									$ 90.73	$ 75.00	$ 73.41
Current share repurchase authorization outstanding	5,000,000								5,000,000
Designated 6.75% Series C cumulative redeemable preferred shares
Authorized Shares [Line Items]
Total authorized shares									0.0	11,600,000
Preferred Shares [Abstract]
Date of issuance										May 08, 2003
Number of preferred shares issued					11,600,000					11,600,000
Annual dividend rate										6.75%
Total consideration										280,900,000
Underwriting discounts and commissions										9,100,000
Aggregate liquidation value					290,000,000					290,000,000
Distribution on liquidation	$ 25.00								$ 25.00
Redemption date									Mar. 18, 2003
Designated 6.5% Series D cumulative redeemable preferred shares
Authorized Shares [Line Items]
Total authorized shares									9,200,000	9,200,000
Preferred Shares [Abstract]
Date of issuance									Nov. 09, 2004
Number of preferred shares issued	9,200,000								9,200,000
Annual dividend rate									6.50%
Total consideration									222,300,000
Underwriting discounts and commissions									7,700,000
Aggregate liquidation value	230,000,000								230,000,000
Distribution on liquidation	$ 25.00								$ 25.00
Designated 7.25% Series E cumulative redeemable preferred shares
Authorized Shares [Line Items]
Total authorized shares									15,000,000	15,000,000
Preferred Shares [Abstract]
Date of issuance									Jun. 08, 2011
Number of preferred shares issued	15,000,000								15,000,000
Annual dividend rate									7.25%
Total consideration									361,700,000
Underwriting discounts and commissions									12,100,000
Aggregate liquidation value	373,800,000								373,800,000
Distribution on liquidation	$ 25.00								$ 25.00
Designated 5.875% Series F non-cumulative redeemable preferred shares
Authorized Shares [Line Items]
Total authorized shares									10,000,000	0.0
Preferred Shares [Abstract]
Date of issuance									Feb. 14, 2013
Number of preferred shares issued	10,000,000								10,000,000
Annual dividend rate									5.875%
Total consideration									242,300,000
Underwriting discounts and commissions									7,700,000
Aggregate liquidation value	$ 250,000,000								$ 250,000,000
Distribution on liquidation	$ 25.00								$ 25.00
Designated and redeemed preference shares [Member]
Authorized Shares [Line Items]
Total authorized shares									25,600,000	14,000,000
Undesignated [Member]
Authorized Shares [Line Items]
Total authorized shares									10,200,000	20,200,000