Retirement Plans - Additional Information (Detail) $ in Millions	6 Months Ended		12 Months Ended
	Jun. 30, 2016 USD ($) RetirementPlans	Jun. 30, 2015 USD ($)	Dec. 31, 2008
Retirement Plans Disclosure [Abstract]
Number of supplemental non-qualified retirement plans sponsored by the Corporation | RetirementPlans	2
Consecutive calendar years used to calculate BRP benefits			5 years
Term of employment for consideration of BRP			10 years
Percent of employer match	100.00%
Employee contribution percentage	6.00%
Additional discretionary contribution, performance goals, percentage	3.00%
Contribution expense of the Corporation | $	$ 4.7	$ 4.0