Common and Preferred Stock (Detail) - USD ($)					1 Months Ended	12 Months Ended
	Aug. 03, 2017	Jun. 19, 2015	Jan. 01, 2012	Jun. 29, 2009	Dec. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2010	Jun. 30, 2025	Sep. 30, 2022	Dec. 31, 2008	Dec. 31, 2007
Common Stock [Line Items]
Net proceeds from issuance of common stock					$ 106,400,000	$ 8,207,000	$ 112,085,000	$ 7,898,000
Shares of common stock authorized (in shares)					450,000,000	450,000,000	450,000,000
Maximum voluntary optional cash payments made by shareholders to purchase common shares				$ 100,000
Warrant exercise period			10 years
Number of common stock issued upon exercise of warrant (in shares)			489,000
Exercise price of warrant (in usd per share)			$ 11.30
Dividend Reinvestment Plan
Common Stock [Line Items]
Number of shares reissued from treasury stock (in shares)						713,000	554,000	713,000
Proceeds from treasury stock reissued						$ 8,200,000	$ 5,200,000	$ 7,000,000
Parent Company
Common Stock [Line Items]
Net proceeds from issuance of common stock						$ 8,207,000	$ 112,085,000	$ 7,898,000
Parent Company | Repurchase Plan
Common Stock [Line Items]
Amount authorized under share repurchase program													$ 4,700,000
Number of shares repurchased (in shares)						0	0	0
Parent Company | Other Stock Repurchases
Common Stock [Line Items]
Number of shares repurchased (in shares)						218,000	328,000	387,000
Average price of shares purchased (in usd per share)						$ 12.12	$ 9.73	$ 9.95
Parent Company | U.S. Treasury Warrants
Common Stock [Line Items]
Exercise price of warrant (in usd per share)									$ 16.12
Amount of shares each warrant entitles buyer to purchase (in shares)									1.1030
Parent Company | Warrants | U.S. Treasury Warrants
Common Stock [Line Items]
Value of warrants to purchase common stock												$ 2,500,000
Common Stock
Common Stock [Line Items]
Stock issued (in shares)					9,240,000
Series A Preferred Stock
Common Stock [Line Items]
Stock issued (in shares)		4,600,000
Liquidation preference per share (in usd per share)		$ 25
Fixed dividend rate per annum		6.25%
Proceeds from issuance of preferred stock, net of issuance costs		$ 111,600,000
Series A Preferred Stock | LIBOR | Forecast
Common Stock [Line Items]
Spread on variable dividend rate per annum										3.85%
Series B Preferred Stock
Common Stock [Line Items]
Stock issued (in shares)	4,000,000
Liquidation preference per share (in usd per share)	$ 25
Fixed dividend rate per annum	550.00%
Proceeds from issuance of preferred stock, net of issuance costs	$ 98,100,000
Series B Preferred Stock | LIBOR | Forecast
Common Stock [Line Items]
Spread on variable dividend rate per annum											357.80%