Dividends (Details) - $ / shares			12 Months Ended
	Sep. 29, 2017	Nov. 02, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Class of Stock [Line Items]
Common shares, declared		$ 1	$ 3.960	$ 3.840	$ 3.620
Common shares, paid			3.940	3.800	3.550
Preferred shares, paid			1.354	1.354	1.354
Common shares, ordinary dividend			3.940	3.800	3.515
Common shares, capital gain			0.000	0.000	0.035
5.417% Series 1 Cumulative Convertible Preferred shares, ordinary dividend			1.354	1.354	1.340
5.417% Series 1 Cumulative Convertible Preferred shares, capital gain			$ 0.000	$ 0.000	$ 0.014
Common Stock, Ordinary Dividends, Tax Rate			15.00%	15.00%	15.00%
Preferred Stock, Ordinary Dividends, Tax Rate			15.00%	15.00%	15.00%
5.417% Series 1 Cumulative Convertible Preferred Shares
Class of Stock [Line Items]
Preferred shares, percentage			5.417%	5.417%	5.417%
Preferred shares, declared			$ 1.354	$ 1.354	$ 1.354
Preferred shares, paid			$ 1.354	$ 1.354	$ 1.354
5.0% Series C Cumulative Redeemable Preferred Shares
Class of Stock [Line Items]
Preferred shares, percentage	5.00%		5.00%	5.00%	5.00%
Preferred shares, declared			$ 0.368	$ 0.000	$ 0.000
Preferred shares, paid			$ 0.000	$ 0.000	$ 0.000