Shareholders' Equity - Issued and Outstanding Preferred Stock (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Class of Stock [Line Items]
Carrying value	$ 879,010	$ 872,500
Series A Fixed Rate Cumulative Perpetual Preferred Stock [Member]
Class of Stock [Line Items]
Preferred stock, shares issued and outstanding	230,000
Carrying value	229,236	226,965
Series C Fixed Rate Cumulative Perpetual Preferred Stock [Member]
Class of Stock [Line Items]
Preferred stock, shares issued and outstanding	151,500
Carrying value	149,774	145,535
Series D Fixed Rate Non-Cumulative Perpetual Preferred Stock [Member]
Class of Stock [Line Items]
Preferred stock, shares issued and outstanding	50,000
Carrying value	$ 500,000	$ 500,000