Postretirement Benefits - Schedule of Amounts Recognized in Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net amounts recognized at year end	$ (152,149)	$ (152,999)	$ (97,467)	$ (168,129)
Pension Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net loss (gain)	145,623	146,571
Prior service cost	5,088	3,362
Net amounts recognized at year end	150,711	149,933
Other Benefits [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net loss (gain)	1,447	3,066
Net amounts recognized at year end	$ 1,447	$ 3,066