Shareholders' Equity (Narrative) (Details)										1 Months Ended		3 Months Ended								12 Months Ended
	Dec. 29, 2017 $ / shares	Dec. 15, 2017 $ / shares	Sep. 29, 2017 $ / shares	Sep. 15, 2017 $ / shares	Jun. 30, 2017 $ / shares	Jun. 15, 2017 $ / shares	Mar. 31, 2017 $ / shares	Mar. 15, 2017 $ / shares	Jun. 27, 2013 USD ($) shares	May 31, 2013 USD ($) $ / shares shares	Mar. 31, 2004 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) director period $ / shares shares	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($) $ / shares shares	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2017 USD ($) director period $ / shares shares	Dec. 31, 2016 USD ($) $ / shares shares	Dec. 31, 2015 USD ($) $ / shares shares	Nov. 10, 2017 USD ($)
Class of Stock [Line Items]
Aggregate authorized shares (in shares) | shares												325,000,000								325,000,000
Authorized common shares (in shares) | shares												225,000,000								225,000,000
Authorized preference shares (in shares) | shares												100,000,000								100,000,000
Dividends declared per common share																				$ 1.28	$ 1.24	$ 1.2
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 usd per share)												$ 1				$ 1				$ 1	$ 1
Preference shares, shares outstanding (in shares) | 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		$ 0.32		$ 0.32		$ 0.32		$ 0.32
Dividends per common share (in usd per share)	$ 0.32		$ 0.32		$ 0.32		$ 0.32
Share repurchase program, authorized aggregate amount | $																							$ 500,000,000
Repurchase of shares (in shares) | shares																				1,300,000
Common shares repurchased during period, aggregate value | $																				$ 188,600,000
Common shares repurchased during period, average cost per share (in usd per share)																				$ (142.67)
Remaining authorized shares per share repurchase program (in shares) | $												$ 500,000,000								$ 500,000,000
Series C Preferred Stock
Class of Stock [Line Items]
Preference shares, dividend rate																				6.08%
Liquidation preference (in usd per share)												$ 1.52								$ 1.52
Quarterly liquidation preference (in usd per share)												0.38								$ 0.38
Preference shares, issued (in shares) | $											$ 250,000,000
Preference shares, shares issued (in shares) | shares											10,000,000
Preference shares, par value (in usd per share)											$ 25
Preference shares, shares redeemed (in shares) | shares									5,000,000
Preference shares, redemption amount | $									$ 125,000,000
Preference shares, shares outstanding (in shares) | shares									5,000,000
Series E Preferred Stock
Class of Stock [Line Items]
Preference shares, dividend rate																				5.375%
Liquidation preference (in usd per share)												1.34375								$ 1.34375
Quarterly liquidation preference (in usd per share)												0.3359375								0.3359375
Preference shares, issued (in shares) | $										$ 275,000,000
Preference shares, shares issued (in shares) | shares										11,000,000
Preference shares, par value (in usd per share)										$ 25
Preference shares, redemption price per share (in usd per share)												$ 25								$ 25
Common shares
Class of Stock [Line Items]
Repurchase of shares (in shares) | shares																				1,322,000	2,741,000	2,473,000
Common shares repurchased during period, aggregate value | $																				$ 1,322,000	$ 2,741,000	$ 2,473,000
Preference shares, shares issued (in shares) | shares																				0	0	7,435,000
Retained earnings
Class of Stock [Line Items]
Dividends on common shares | $																				$ (51,370,000)	$ (51,583,000)	$ (53,967,000)
Dividends on preference shares | $																				$ 22,381,000	$ 22,381,000	$ 22,381,000