Shareholders' Equity - Issued and Outstanding Preferred Stock (Parenthetical) (Detail) - $ / shares	9 Months Ended
	Sep. 30, 2015	Dec. 31, 2014
Series A Fixed Rate Cumulative Perpetual Preferred Stock [Member]
Class of Stock [Line Items]
Preferred stock, liquidation preference per share	$ 1,000	$ 1,000
Preferred stock dividend rate	6.375%
Exercise price of each class of warrants or rights outstanding	$ 73.86
Warrants and rights outstanding	719,175	721,490
Warrants, year of expiration	2018
Series C Fixed Rate Cumulative Perpetual Preferred Stock [Member]
Class of Stock [Line Items]
Preferred stock, liquidation preference per share	$ 1,000	$ 1,000
Series D Fixed Rate Non-Cumulative Perpetual Preferred Stock [Member]
Class of Stock [Line Items]
Preferred stock, liquidation preference per share	$ 10,000	10,000
Preferred stock dividend rate	6.875%
Preferred Stock, Redemption Feature, Redemption Term	90 days
Preferred shares redemption date	Jun. 15, 2016
Series E Fixed-to-Floating Rate Non-Cumulative Perpetual Preferred Stock [Member]
Class of Stock [Line Items]
Preferred stock, liquidation preference per share	$ 1,000	$ 1,000
Preferred stock dividend rate	6.45%
Preferred Stock, Redemption Feature, Redemption Term	90 days
Preferred shares redemption date	Feb. 15, 2024
Date of change in the dividend rate	Feb. 14, 2024
London Interbank offered rate plus basis points	3.61%