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Employee Benefit Plans - Summary of Components of Net Periodic Benefit Cost and Other Changes in Plan Assets and Benefit Obligations Recognized in Other Comprehensive Income (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Other changes in plan assets and benefit obligations recognized in other comprehensive income:      
Tax (benefit) expense related to benefit plans included in other comprehensive income $ 1,100 $ (17,800) $ 32,500
Defined benefit pension plans      
Net periodic benefit cost:      
Service cost 6,105 7,515 12,982
Interest cost 26,007 27,708 25,580
Expected return on plan assets (29,850) (31,833) (29,786)
Amortization of prior service credit (465) (1,037) (1,057)
Recognized actuarial loss 15,168 15,771 21,193
Settlement loss 6,453 0 1,835
Curtailment gain (8,639) 0 0
Net periodic benefit cost 14,779 18,124 30,747
Other changes in plan assets and benefit obligations recognized in other comprehensive income:      
Net actuarial (gain) loss (8,813) 48,502 (95,744)
Prior service cost 8,691 20 0
Amortization of prior service credit 465 1,037 1,057
Total recognized in other comprehensive income 343 49,559 (94,687)
Total recognized in net periodic benefit cost and other comprehensive income 15,122 67,683 (63,940)
Postretirement benefit plans      
Net periodic benefit cost:      
Service cost 54 67 82
Interest cost 1,194 1,377 1,585
Expected return on plan assets 0 0 0
Amortization of prior service credit 0 0 0
Recognized actuarial loss 148 17 578
Settlement loss 0 0 0
Curtailment gain 0 0 0
Net periodic benefit cost 1,396 1,461 2,245
Other changes in plan assets and benefit obligations recognized in other comprehensive income:      
Net actuarial (gain) loss (3,130) (2,205) 3,470
Prior service cost 0 0 0
Amortization of prior service credit 0 0 0
Total recognized in other comprehensive income (3,130) (2,205) 3,470
Total recognized in net periodic benefit cost and other comprehensive income $ (1,734) $ (744) $ 5,715