Postretirement Benefits - Pension Assets by Components at Fair Value (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 307,596	$ 322,898
Total percentage	100.00%	100.00%
Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 143,855	$ 140,762
Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	163,741	182,136
Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 123,428	$ 127,568
Total percentage	40.00%	39.00%
Equity Securities [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 123,420	$ 127,568
Equity Securities [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	8
Corporate Bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 146,381	$ 166,825
Total percentage	47.00%	52.00%
Corporate Bonds [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 146,381	$ 166,825
Other Bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 270	$ 284
Total percentage	0.00%	0.00%
Other Bonds [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 270	$ 284
Guaranteed Annuity Contract [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 17,082	$ 15,027
Total percentage	6.00%	5.00%
Guaranteed Annuity Contract [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 17,082	$ 15,027
Short-term Investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 15,593	$ 9,038
Total percentage	5.00%	3.00%
Short-term Investments [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 15,593	$ 9,038
Other [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 4,842	$ 4,156
Total percentage	2.00%	1.00%
Other [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 4,842	$ 4,156
Financial [Member] | Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 49,391	$ 45,790
Total percentage	16.00%	14.00%
Financial [Member] | Equity Securities [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 49,391	$ 45,790
Financial [Member] | Corporate Bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 36,266	$ 40,889
Total percentage	12.00%	13.00%
Financial [Member] | Corporate Bonds [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 36,266	$ 40,889
Consumer, Cyclical [Member] | Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 24,264	$ 26,542
Total percentage	8.00%	8.00%
Consumer, Cyclical [Member] | Equity Securities [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 24,264	$ 26,542
Technology [Member] | Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 19,871	$ 16,965
Total percentage	6.00%	5.00%
Technology [Member] | Equity Securities [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 19,871	$ 16,965
Industrial [Member] | Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 15,176	$ 6,377
Total percentage	5.00%	2.00%
Industrial [Member] | Equity Securities [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 15,176	$ 6,377
Consumer, Non-Cyclical [Member] | Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 12,216	$ 11,665
Total percentage	4.00%	4.00%
Consumer, Non-Cyclical [Member] | Equity Securities [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 12,216	$ 11,665
Energy [Member] | Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		$ 10,192
Total percentage		3.00%
Energy [Member] | Equity Securities [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		$ 10,192
Energy [Member] | Corporate Bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 25,890	$ 30,936
Total percentage	8.00%	10.00%
Energy [Member] | Corporate Bonds [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 25,890	$ 30,936
Communications [Member] | Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		$ 9,322
Total percentage		3.00%
Communications [Member] | Equity Securities [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		$ 9,322
Other Sectors [Member] | Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 2,510	$ 715
Total percentage	1.00%	0.00%
Other Sectors [Member] | Equity Securities [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 2,502	$ 715
Other Sectors [Member] | Equity Securities [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	8
Utilities [Member] | Corporate Bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 43,229	$ 48,510
Total percentage	14.00%	15.00%
Utilities [Member] | Corporate Bonds [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 43,229	$ 48,510
Other corporates [Member] | Corporate Bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 40,996	$ 46,490
Total percentage	13.00%	14.00%
Other corporates [Member] | Corporate Bonds [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 40,996	$ 46,490