Shareholders' Equity (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
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	500,000	5,200,000	7,700,000
Total cost of shares repurchased	$ 59,300	$ 551,400	$ 695,300
Average cost per share of shares repurchased	$ 112.89	$ 106.30	$ 90.73
Current share repurchase authorization outstanding	2,900,000
Common shares held in treasury, shares	39,303,068	39,400,936
Common Shares Reissued Out Of Treasury	600,000
Total Cost Of Shares Reissued	$ 51,600
Average Cost Per Share Of Shares Reissued	$ 82.88
Preferred Shares [Abstract]
Number of preferred shares issued	34,150,000	34,150,000
Total consideration	$ 0	$ 0	$ 241,265
Loss on redemption of preferred shares	$ 0	$ 0	$ 9,135
Common Shares [Member]
Authorized Shares [Abstract]
Total authorized shares	100,000,000	100,000,000
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
Underwriting discounts and commissions			9,100
Aggregate liquidation value.			$ 290,000
Redemption date			Mar. 18, 2013
Loss on redemption of preferred shares			$ 9,100
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
Underwriting discounts and commissions	7,700
Aggregate liquidation value.	$ 230,000
Distribution on liquidation	$ 25.00
Designated 7.25% Series E cumulative redeemable preferred shares
Authorized Shares [Abstract]
Total authorized shares	17,000,000	17,000,000
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
Underwriting discounts and commissions	12,100
Aggregate liquidation value.	$ 373,800
Distribution on liquidation	$ 25.00
Designated 5.875% Series F non-cumulative redeemable preferred shares
Authorized Shares [Abstract]
Total authorized shares	14,000,000	14,000,000
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
Underwriting discounts and commissions	7,700
Aggregate liquidation value.	$ 250,000
Distribution on liquidation	$ 25.00
Designated and redeemed preference shares [Member]
Authorized Shares [Abstract]
Total authorized shares	26,000,000	26,000,000
Undesignated [Member]
Authorized Shares [Abstract]
Total authorized shares	33,800,000	33,800,000