Shareholders' Equity - Summary of Perpetual Preferred Stock Issued and Outstanding (Parenthetical) (Detail) - $ / shares	9 Months Ended
	Sep. 30, 2016	Dec. 31, 2015
Class of Stock [Line Items]
Shares Issued	420,282
Preferred Stock	$ 0.01	$ 0.01
Series A Preferred Stock [Member]
Class of Stock [Line Items]
Shares Issued	30,000
Preferred Stock	$ 0.01
Series B Preferred Stock [Member]
Class of Stock [Line Items]
Shares Issued	32,000
Preferred Stock	$ 0.01
Series C Preferred Stock [Member]
Class of Stock [Line Items]
Shares Issued	8,000
Preferred Stock	$ 0.01
Series D Preferred Stock [Member]
Class of Stock [Line Items]
Shares Issued	54,000
Preferred Stock	$ 0.01
Series E Preferred Stock [Member]
Class of Stock [Line Items]
Shares Issued	7,667
Preferred Stock	$ 0.01
Series F Preferred Stock [Member]
Class of Stock [Line Items]
Shares Issued	1,615
Preferred Stock	$ 0.01
Series I Preferred Stock [Member]
Class of Stock [Line Items]
Shares Issued	34,000
Preferred Stock	$ 0.01
Series J Preferred Stock [Member]
Class of Stock [Line Items]
Shares Issued	40,000
Preferred Stock	$ 0.01
Series K Preferred Stock [Member]
Class of Stock [Line Items]
Shares Issued	28,000
Preferred Stock	$ 0.01
Series L Preferred Stock [Member]
Class of Stock [Line Items]
Shares Issued	52,000
Preferred Stock	$ 0.01
Series M Preferred Stock [Member]
Class of Stock [Line Items]
Shares Issued	80,000
Preferred Stock	$ 0.01
Series N Preferred Stock [Member]
Class of Stock [Line Items]
Shares Issued	27,000
Preferred stock dividend rate	6.30%
Preferred Stock	$ 0.01
Series O Preferred Stock [Member]
Class of Stock [Line Items]
Shares Issued	26,000
Preferred stock dividend rate	5.30%
Preferred Stock	$ 0.01