Retirement Benefit Plans (Fair Values of Pension Plan Assets) (Details) - Pension Benefits [Member] - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 564,365	$ 515,555	$ 491,550
Cash And Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	5,496	15,568
Total Cash and Cash-Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	5,496	15,568
Fixed Income Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	113,331	136,085
Mutual Fund-Fixed Income [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	31,839	48,805
Total Fixed Income [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	145,170	184,890
Domestic And Foreign Stock [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	102,422	104,461
Mutual Funds-Equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	168,299	157,630
Total Equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	270,721	262,091
Real Assets Fund [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		27,690
Total Real Assets [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	421,387	490,239
Common Trust Fund (Fixed Income) [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	114,068	25,316
Common Trust Fund (Real Assets) [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	28,910
Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	308,050	354,148
Level 1 [Member] | Cash And Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	5,496	15,568
Level 1 [Member] | Total Cash and Cash-Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	5,496	15,568
Level 1 [Member] | Mutual Fund-Fixed Income [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	31,839	48,805
Level 1 [Member] | Total Fixed Income [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	31,839	48,805
Level 1 [Member] | Domestic And Foreign Stock [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	102,416	104,455
Level 1 [Member] | Mutual Funds-Equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	168,299	157,630
Level 1 [Member] | Total Equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	270,715	262,085
Level 1 [Member] | Real Assets Fund [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		27,690
Level 1 [Member] | Total Real Assets [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	308,050	354,148
Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	113,175	136,091
Level 2 [Member] | Fixed Income Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	113,169	136,085
Level 2 [Member] | Total Fixed Income [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	113,169	136,085
Level 2 [Member] | Domestic And Foreign Stock [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	6	6
Level 2 [Member] | Total Equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	6	6
Level 2 [Member] | Total Real Assets [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	113,175	$ 136,091
Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	162
Level 3 [Member] | Fixed Income Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	162
Level 3 [Member] | Total Fixed Income [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	162
Level 3 [Member] | Total Real Assets [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 162