Retirement Benefit Plans (Fair Values of Pension Plan Assets) (Details) - Pension Benefits [Member] - USD ($)	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	$ 515,555,000	$ 491,550,000	$ 438,708,000
Cash And Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	15,568,000	44,224,000
Total Cash and Cash-Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets		44,224,000
Fixed Income Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	136,085,000	105,721,000
Mutual Fund-Fixed Income [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	48,805,000	47,453,000
Total Fixed Income [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	184,890,000	153,174,000
Domestic And Foreign Stock [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	104,461,000	97,905,000
Mutual Funds-Equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	157,630,000	147,051,000
Total Equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	262,091,000	244,956,000
Real Assets Fund [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	27,690,000	24,653,000
Total Real Assets [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	490,239,000	467,007,000
Common Trust Fund (Fixed Income) [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	25,316,000	24,543,000
Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	354,148,000	361,276,000
Level 1 [Member] | Cash And Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	15,568,000	44,224,000
Level 1 [Member] | Total Cash and Cash-Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets		44,224,000
Level 1 [Member] | Mutual Fund-Fixed Income [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	48,805,000	47,453,000
Level 1 [Member] | Total Fixed Income [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	48,805,000	47,453,000
Level 1 [Member] | Domestic And Foreign Stock [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	104,455,000	97,895,000
Level 1 [Member] | Mutual Funds-Equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	157,630,000	147,051,000
Level 1 [Member] | Total Equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	262,085,000	244,946,000
Level 1 [Member] | Real Assets Fund [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	27,690,000	24,653,000
Level 1 [Member] | Total Real Assets [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	354,148,000	361,276,000
Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	136,091,000	105,731,000
Level 2 [Member] | Fixed Income Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	136,085,000	105,721,000
Level 2 [Member] | Total Fixed Income [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	136,085,000	105,721,000
Level 2 [Member] | Domestic And Foreign Stock [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	6,000	10,000
Level 2 [Member] | Total Equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	6,000	10,000
Level 2 [Member] | Total Real Assets [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	136,091,000	105,731,000
Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	$ 0	$ 0