Deferred Compensation Plan - Summary of Activity of Vested Awards (Detail) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012
Individual Director's Awards [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vested, Share Award, Beginning balance	876.0	908.0
Share based dividend, Share Award	34.5	26.7
Vested during the year, Share Award	252.0	152.0
Redeemed vested awards, Share Award	(103.8)	(58.7)
Converted to Redeemable Common Shares, Share Award	(252.0)	(152.0)
Vested, Share Award, Ending balance	806.7	876.0
Vested, Weighted Average Grant Date Fair Value, Beginning balance	$ 815	$ 702
Share based dividend, Weighted Average Grant Date Fair Value	$ 828	$ 715
Vested during the year, Weighted Average Grant Date Fair Value	$ 881	$ 812
Redeemed vested awards, Weighted average grant date fair value	$ 794	$ 682
Converted to Redeemable Common Shares, Weighted Average Grant Date Fair Value	$ 881	$ 812
Vested, Weighted Average Grant Date Fair Value, Ending balance	$ 886	$ 815
Key Employee Awards [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vested, Share Award, Beginning balance	339.7	256.6
Share based dividend, Share Award	13.4	7.5
Vested during the year, Share Award	74.2	78.6
Redeemed vested awards, Share Award	(14.8)
Converted to Redeemable Common Shares, Share Award	(4.0)	(3.0)
Vested, Share Award, Ending balance	408.5	339.7
Vested, Weighted Average Grant Date Fair Value, Beginning balance	$ 840	$ 710
Share based dividend, Weighted Average Grant Date Fair Value	$ 828	$ 715
Vested during the year, Weighted Average Grant Date Fair Value	$ 1,015	$ 892
Redeemed vested awards, Weighted average grant date fair value	$ 814
Converted to Redeemable Common Shares, Weighted Average Grant Date Fair Value	$ 886	$ 812
Vested, Weighted Average Grant Date Fair Value, Ending balance	$ 920	$ 840