CONSOLIDATED BALANCE SHEETS (Parenthetical) - $ / shares	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Common shares, par value	$ 0.10	$ 0.10
Common shares, shares authorized	600,000,000	600,000,000
Common shares, shares issued	369,344,148	368,512,410
Treasury at cost	584,236	612,629
Class A Cumulative Redeemable Preferred Shares [Member]
Cumulative redeemable preferred shares, liquidation value	$ 500	$ 500
Cumulative redeemable preferred shares, shares authorized	750,000	750,000
Cumulative redeemable preferred shares, shares issued	350,000	350,000
Cumulative redeemable preferred shares, shares outstanding	350,000	350,000
Preferred stock dividend rate	6.375%	6.375%
Cumulative redeemable preferred shares, par value
Class J Cumulative Redeemable Preferred Shares [Member]
Cumulative redeemable preferred shares, liquidation value	$ 500	$ 500
Cumulative redeemable preferred shares, shares authorized	750,000	750,000
Cumulative redeemable preferred shares, shares issued	400,000	400,000
Cumulative redeemable preferred shares, shares outstanding	400,000	400,000
Preferred stock dividend rate	6.50%	6.50%
Cumulative redeemable preferred shares, par value
Class K Cumulative Redeemable Preferred Shares [Member]
Cumulative redeemable preferred shares, liquidation value	$ 500	$ 500
Cumulative redeemable preferred shares, shares authorized	750,000	750,000
Cumulative redeemable preferred shares, shares issued	300,000	300,000
Cumulative redeemable preferred shares, shares outstanding	300,000	300,000
Preferred stock dividend rate	6.25%	6.25%
Cumulative redeemable preferred shares, par value