Pension and Other Postretirement Employee Benefit Plans Components of Net Periodic Cost of Pension and Other Postretirement Employee Benefit Plans (Detail 2) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Service Cost					$ 1,800
Net periodic cost					$ 5,300
Pension Benefit Plans
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Service Cost	$ 474	$ 382	$ 1,034	$ 780
Interest cost	3,296	3,633	6,574	7,261
Expected return on plan assets	(4,688)	(4,878)	(9,382)	(9,761)
Amortization of prior service cost (credit)	2	6	4	11
Amortization of actuarial loss	2,412	2,761	4,937	5,645
Net periodic cost	1,496	1,904	3,167	3,936
Other Postretirement Employee Benefit Plans
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Service Cost	13	51	81	125
Interest cost	667	810	1,371	1,576
Expected return on plan assets	(1)	(1)	(1)	(1)
Amortization of prior service cost (credit)	(384)	(428)	(767)	(856)
Amortization of actuarial loss	(1,821)	(2,058)	(3,301)	(3,441)
Net periodic cost	$ (1,526)	$ (1,626)	$ (2,617)	$ (2,597)