Employee Benefit Plans - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation	$ 52,419,000	$ 49,412,000
Contributions to pension plans	0	0
Cost incurred for providing different benefit	3,073,000	3,454,000	4,597,000
ESI Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Contributions to pension plans	0	0
ESI Excess Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Contributions to pension plans	0	0
Equity securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets target allocations, minimum	30.00%
Plan assets target allocations, maximum	70.00%
Fixed income securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets target allocations, minimum	20.00%
Plan assets target allocations, maximum	60.00%
Cash and cash equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets target allocations, minimum	0.00%
Plan assets target allocations, maximum	50.00%
401 (k) Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Employer contribution to ESI plan	On July 1, 2013, we changed the rate at which we made contributions to the ESI 401(k) Plan on behalf of our employees. Prior to July 1, 2013, we contributed 100% of the first 1% and 50% of the next 4% of an employee’s salary that the employee contributed to his or her ESI 401(k) Plan account. Beginning July 1, 2013, we contribute 50% of the first 6% of an employee’s salary that the employee contributes to his or her ESI 401(k) Plan account.
Scenario, Forecast [Member]
Defined Benefit Plan Disclosure [Line Items]
Estimated net actuarial loss expected to be amortized from accumulated other comprehensive income and recognized in net periodic pension benefit cost				0
Estimated prior service cost credit expected to be amortized from accumulated other comprehensive income and recognized in net periodic pension benefit cost				$ 1,555,000
1% of Employee [Member] | Prior To July 1, 2013 [Member] | 401 (k) Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Contribution made for employees towards ESI		100.00%
4% of Employee [Member] | Prior To July 1, 2013 [Member] | 401 (k) Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Contribution made for employees towards ESI		50.00%
6% of Employee [Member] | Beginning From July 1, 2013 [Member] | 401 (k) Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Contribution made for employees towards ESI		50.00%