Shareholders' Equity (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Authorized Shares [Abstract]
Total authorized shares	200,000,000	200,000,000
Par value per share	$ 1.00	$ 1.00
Common Shares [Abstract]
Number of shares repurchased	5,200,000	7,700,000	7,100,000
Total cost of shares repurchased	$ 551,409,000	$ 695,304,000	$ 532,927,000
Average cost per share of shares repurchased	$ 106.30	$ 90.73	$ 75.00
Current share repurchase authorization outstanding	3,400,000
Common shares held in treasury, shares	39,400,936	34,213,611
Preferred Shares [Abstract]
Number of preferred shares issued	34,150,000	34,150,000
Total consideration	0	241,265,000	0
Common Shares [Member]
Authorized Shares [Abstract]
Total authorized shares	130,000,000	130,000,000
Designated 6.75% Series C cumulative redeemable preferred shares [Member]
Preferred Shares [Abstract]
Date of issuance		May 08, 2003
Number of preferred shares issued		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
Redemption date		Mar. 18, 2013
Designated 6.5% Series D cumulative redeemable preferred shares
Authorized Shares [Abstract]
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
Annual dividend rate	6.50%
Total consideration	222,300,000
Underwriting discounts and commissions	7,700,000
Aggregate liquidation value.	230,000,000
Distribution on liquidation	$ 25.00
Designated 7.25% Series E cumulative redeemable preferred shares
Authorized Shares [Abstract]
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
Annual dividend rate	7.25%
Total consideration	361,700,000
Underwriting discounts and commissions	12,100,000
Aggregate liquidation value.	373,800,000
Distribution on liquidation	$ 25.00
Designated 5.875% Series F non-cumulative redeemable preferred shares
Authorized Shares [Abstract]
Total authorized shares	10,000,000	10,000,000
Preferred Shares [Abstract]
Date of issuance	Feb. 14, 2013
Number of preferred shares issued	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
Distribution on liquidation	$ 25.00
Designated and redeemed preference shares [Member]
Authorized Shares [Abstract]
Total authorized shares	25,600,000	25,600,000
Undesignated [Member]
Authorized Shares [Abstract]
Total authorized shares	10,200,000	10,200,000