Pension and Other Retirement Plans - Pension Plan Assets by Asset Category (Details) - Pension Plans - USD ($) $ in Thousands		Dec. 31, 2017		Dec. 31, 2016		Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets		$ 1,660,001		$ 1,545,862		$ 1,530,898
Total pension plan assets measured at fair value		1,274,569		1,096,386
Defined benefit plan, fair value of plan assets, measured using net asset value		360,382	[1]	424,875	[2]
Contract value of pension plan assets		25,050		24,601
Level 1
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets measured at fair value		704,141		575,884
Level 2
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets measured at fair value		570,428		520,502
Level 3
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets measured at fair value		0		0
Registered investment companies
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets		704,141		575,884
Registered investment companies | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets		704,141		575,884
Registered investment companies | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets		0		0
Registered investment companies | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets		0		0
Common/collective trusts
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets		12,607		16,060
Common/collective trusts | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets		0		0
Common/collective trusts | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets		12,607		16,060
Common/collective trusts | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets		0		0
U.S. government securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets		250,792		200,782
U.S. government securities | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets		0		0
U.S. government securities | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets		250,792		200,782
U.S. government securities | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets		0		0
Corporate bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets		294,362		264,451
Corporate bonds | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets		0		0
Corporate bonds | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets		294,362		264,451
Corporate bonds | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets		0		0
Mortgage-backed and asset-backed securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets		31,188		30,961
Mortgage-backed and asset-backed securities | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets		0		0
Mortgage-backed and asset-backed securities | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets		31,188		30,961
Mortgage-backed and asset-backed securities | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets		0		0
Other alternative investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets		(35,330)	[3]	(9,155)	[4]
Other alternative investments | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets		0	[3]	0	[4]
Other alternative investments | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets		(35,330)	[3]	(9,155)	[4]
Other alternative investments | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets		0	[3]	0	[4]
Pooled separate account
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets		16,809		17,403
Pooled separate account | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets		0		0
Pooled separate account | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets		16,809		17,403
Pooled separate account | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets		0		0
Pending net purchases
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	[4]	$ (89,000)		$ (58,000)

[1] (2)Certain common/collective trusts, the 103-12 investment entity, the international equity limited liability company, real estate, private equity and venture capital limited partnerships that are measured at fair value using the NAV per share practical expedient have not been categorized in the fair value hierarchy. The fair value amounts presented in the table above are intended to permit reconciliation of the fair value hierarchy to the total value of plan assets.

[2] (2)Certain common/collective trusts, the 103-12 investment entity, the international equity limited partnership, real estate, private equity and venture capital limited partnerships that are measured at fair value using the NAV per share practical expedient have not been categorized in the fair value hierarchy. The fair value amounts presented in the table above are intended to permit reconciliation of the fair value hierarchy to the total value of plan assets.

[3] Other includes pending net security purchases of $89 million.

[4] Other includes pending net security purchases of $58 million.