Common Stock - Additional Information (Detail) (USD $)	0 Months Ended		12 Months Ended				0 Months Ended	12 Months Ended
	Jan. 01, 2012	Jun. 29, 2009	Dec. 31, 2008	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Jul. 31, 2008 Warrant	Dec. 31, 2010	Dec. 31, 2007
Common Stock [Line Items]
Common stock, shares authorized				332,023,233	199,629,268
Maximum voluntary optional cash payments made by shareholders to purchase common shares		$ 100,000
Warrant exercise period	10 years		10 years
Number of common stock issued upon exercise of warrant	489,000
Exercise price of warrant (in usd per share)	$ 11.30
Dividend Reinvestment Plan [Member]
Common Stock [Line Items]
Number of shares reissued from treasury stock				499,000	775,000	721,000
Proceeds from treasury stock reissued				$ 5,000,000	$ 7,600,000	$ 8,000,000
Parent Company [Member]
Common Stock [Line Items]
Exercise price of warrant (in usd per share)							$ 15.64	$ 16.12
Warrants issued							918,000
Amount of shares each warrant entitles buyer to purchase							1.2155	1.1030
Expiration date of issued warrants							Jun. 30, 2015	Nov. 14, 2018
Number of shares repurchased				174,000	78,000	55,000
Number of shares authorized to repurchase									4,700,000
Number of shares repurchased during the period				0	0	0
Average price of shares purchased (in usd per share)				$ 9.96	$ 9.91	$ 9.41
Parent Company [Member] | Dividend Reinvestment Plan [Member]
Common Stock [Line Items]
Common stock, shares authorized		10,000,000
Parent Company [Member] | Warrants [Member]
Common Stock [Line Items]
Number of shares repurchased			2,500,000