Pension and Other Postretirement Benefit Plans (Schedule of Fair Value of Plan Assets) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 679,245	$ 632,434
Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	5,214	650
Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Cash/Accrued Income
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	5,202	639
Cash/Accrued Income | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	5,202	639
Cash/Accrued Income | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Cash/Accrued Income | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
US Equities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	12	11
US Equities | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	12	11
US Equities | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
US Equities | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Private Equity Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	674,031	631,784
Private Equity Funds | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Private Equity Funds | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Private Equity Funds | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 0	$ 0