Income Taxes - Summary of Reconciliation of the Income Tax Expense at the Statutory Income Tax Rate and the Effective Tax Rate for Income from Continuing Operations before Income Taxes (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Tax Disclosure [Abstract]
Income from continuing operations (before-tax)	$ 51,138,000	$ 90,027,000	$ 72,889,000
Income tax at the US statutory federal rate (35%)	17,898,000	31,509,000	25,511,000
Rate differential	1,145,000	2,380,000
State income taxes, net of federal benefit	(337,000)	271,000	(8,000)
Dividends paid deduction	(27,873,000)	(33,345,000)	(25,454,000)
Change in valuation allowance	8,988	(697,000)
Other items, net	519,000	608,000	(30,000)
Total income tax expense	$ 340,000	$ 726,000	$ 19,000