Shareholders' Equity (Narrative) (Details)										1 Months Ended		3 Months Ended								12 Months Ended
	Dec. 30, 2016 $ / shares	Sep. 30, 2016 $ / shares	Jun. 30, 2016 $ / shares	Mar. 31, 2016 $ / shares	Mar. 15, 2016 $ / shares	Dec. 15, 2015 $ / shares	Sep. 15, 2015 $ / shares	Jun. 15, 2015 $ / shares	Jun. 27, 2013 USD ($) shares	May 31, 2013 USD ($) $ / shares shares	Mar. 31, 2004 USD ($) $ / shares shares	Dec. 31, 2016 USD ($) director period $ / shares shares	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2015 USD ($) $ / shares shares	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2016 USD ($) director period $ / shares shares	Dec. 31, 2015 USD ($) $ / shares shares	Dec. 31, 2014 USD ($) $ / shares shares	Aug. 02, 2016 USD ($)
Class of Stock [Line Items]
Aggregate authorized shares | shares												325,000,000								325,000,000
Authorized common shares | shares												225,000,000								225,000,000
Authorized preference shares | shares												100,000,000								100,000,000
Dividends declared per common share, in dollars per share | $ / shares																				$ 1.24	$ 1.2	$ 1.16
Dividends on preference shares | $												$ 5,595,000	$ 5,595,000	$ 5,596,000	$ 5,595,000	$ 5,595,000	$ 5,595,000	$ 5,596,000	$ 5,595,000	$ 22,381,000	$ 22,381,000	$ 22,381,000
Preference Shares, Par Value (In dollars per share) | $ / shares												$ 1				$ 1				$ 1	$ 1
Preference shares, Shares outstanding | shares												16,000,000				16,000,000				16,000,000	16,000,000
Preference shares, Amount of preferred dividends in arrears, equivalent number of periods, trigger | period												6								6
Preference shares, Preferred stock dividends in arrears, triggered shareholder election right, number of directors | director												2								2
Common shares
Class of Stock [Line Items]
Dividends declared per common share, in dollars per share | $ / shares					$ 0.31	$ 0.31	$ 0.31	$ 0.31
Dividends per common share, in usd per share | $ / shares	$ 0.31	$ 0.31	$ 0.31	$ 0.31	$ 0.31
Repurchase of shares (in shares) | shares																				2,700,000
Share repurchase program, Authorized aggregate amount | $																							$ 500,000,000
Common shares repurchased during period, Aggregate value | $																				$ 309,400,000
Common shares repurchased during period, Average cost per share (in dollars per share) | $ / shares																				$ 112.87
Remaining authorized shares per share repurchase program | $												$ 500,000,000								$ 500,000,000
Series C Preferred Stock
Class of Stock [Line Items]
Preference shares, issued | $											$ 250,000,000
Preference shares, Shares issued | shares											10,000,000
Preference Shares, Par Value (In dollars per share) | $ / shares											$ 25
Preference shares, Shares redeemed | shares									5,000,000
Preference shares, Redemption amount | $									$ 125,000,000
Preference shares, Shares outstanding | shares									5,000,000
Preference shares, Dividend rate																				6.08%
Series E Preferred Stock
Class of Stock [Line Items]
Preference shares, issued | $										$ 275,000,000
Preference shares, Shares issued | shares										11,000,000
Preference Shares, Par Value (In dollars per share) | $ / shares										$ 25
Preference shares, Redemption price per share | $ / shares												$ 25								$ 25
Preference shares, Dividend rate																				5.375%
Common shares
Class of Stock [Line Items]
Repurchase of shares (in shares) | shares																				2,741,000	2,473,000	5,355,000
Common shares repurchased during period, Aggregate value | $																				$ 2,741,000	$ 2,473,000	$ 5,355,000
Preference shares, Shares issued | shares																				0	7,435,000	0
Retained earnings
Class of Stock [Line Items]
Dividends on common shares | $																				$ 51,583,000	$ 53,967,000	$ 45,912,000
Dividends on preference shares | $																					22,381,000	22,381,000
Common shares repurchased during period, Aggregate value | $																				$ 0	$ 0	$ 497,175,000