Federal Income Taxes - Additional Information (Detail) - USD ($)	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014
Income Tax Disclosure [Abstract]
Income taxes paid or refunded, net	$ 52,106,000	$ 41,121,000
Valuation allowance	0	0
Ordinary loss tax carryforwards subject to expiration	$ 0
Ordinary loss tax carryforwards expiration date	Dec. 31, 2015
Provision for penalties recognized relating to uncertain tax positions	$ 0	0
Interest expense recognized relating to uncertain tax positions	$ 0	$ 0