Shareholders' Equity (Details) - USD ($)					1 Months Ended	3 Months Ended		6 Months Ended
	Jun. 29, 2018	Jun. 15, 2018	Mar. 29, 2018	Mar. 15, 2018	Jun. 30, 2018	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Nov. 10, 2017
Class of Stock [Line Items]
Dividends declared per common share (in dollars per share)		$ 0.33				$ 0.33	$ 0.32	$ 0.66	$ 0.64
Dividends paid per common share (in dollars per share)	$ 0.33		$ 0.33
Dividends declared and paid, preference shares						$ 5,596,000	$ 5,596,000	$ 11,191,000	$ 11,191,000
Shares issued (In shares)					16,010,000	16,010,000		16,010,000		16,000,000
Par value (In dollars per share)					$ 1	$ 1		$ 1		$ 1
Retained Earnings
Class of Stock [Line Items]
Dividends declared and paid, preference shares								$ 11,191,000	11,191,000
Dividends declared and paid, common shares								$ 26,468,000	$ 25,877,000
Common Shares
Class of Stock [Line Items]
Dividends declared per common share (in dollars per share)				$ 0.33
Share repurchase program, authorized amount											$ 500,000,000
Share repurchase program, shares repurchased (in shares)								0
Share repurchase program, remaining authorized aggregate amount					$ 500,000,000	$ 500,000,000		$ 500,000,000
Series C Preference Shares
Class of Stock [Line Items]
Dividend rate, percentage								6.08%
Liquidation preference per annum (in dollars per share)					$ 1.52	$ 1.52		$ 1.52
Liquidation preference quarterly (in dollars per share)					0.38	0.38		$ 0.38
Series E Preference Shares
Class of Stock [Line Items]
Dividend rate, percentage								5.375%
Liquidation preference per annum (in dollars per share)					1.34375	1.34375		$ 1.34375
Liquidation preference quarterly (in dollars per share)					0.3359375	0.3359375		0.3359375
Depositary Shares
Class of Stock [Line Items]
Liquidation preference per annum (in dollars per share)					1.4375	1.4375		1.4375
Liquidation preference quarterly (in dollars per share)					$ 0.359375	$ 0.359375		$ 0.359375
Shares issued (In shares)					10,000,000	10,000,000		10,000,000
Redemption price per share (in dollars per share)					$ 25	$ 25		$ 25
Percentage interest in underlying shares					0.10%	0.10%		0.10%
Depositary Shares | Prior To June 30, 2023
Class of Stock [Line Items]
Redemption price per share (in dollars per share)					$ 26	$ 26		$ 26
Series F Preference Shares
Class of Stock [Line Items]
Dividend rate, percentage					5.75%
Liquidation preference per annum (in dollars per share)					$ 1,437.50	1,437.50		1,437.50
Liquidation preference quarterly (in dollars per share)					$ 359.375	$ 359.375		$ 359.375
Shares issued (In shares)					10,000	10,000		10,000
Par value (In dollars per share)					$ 1	$ 1		$ 1
Liquidation preference (in dollars per share)					25,000	25,000		25,000
Redemption price per share (in dollars per share)					25,000	25,000		25,000
Series F Preference Shares | Prior To June 30, 2023
Class of Stock [Line Items]
Redemption price per share (in dollars per share)					$ 26,000	$ 26,000		$ 26,000