Postretirement Benefit Plans - Additional Information (Details) - USD ($)	6 Months Ended
	Jun. 30, 2017	Dec. 31, 2016
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Pension benefit obligation	$ 548,579,000	$ 527,522,000
Pension Benefits [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Cash contributions made to benefit plans	12,000,000
Pension Benefits [Member] | Maximum [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Expected cash contributions	20,000,000
Funded plan [Member] | Pension Benefits [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Pension benefit obligation	469,357,000	449,613,000
Nonqualified plan [Member] | SERP (Active) [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Pension benefit obligation	76,377,000	74,687,000
Nonqualified plan [Member] | Unfunded Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Insurance cash value and Rabbi Trust investments supporting supplemental pension plan liability	$ 91,000,000	$ 86,000,000