Retirement Benefit Plans - Components of Defined Benefit Obligation (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets: at beginning of year	$ 438,708
Fair value of plan assets, end of year	491,550	$ 438,708
Fair value of plan assets	438,708	438,708		$ 491,550	$ 438,708
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation: at beginning of year	456,911	412,608
Service cost	13,511	12,920	$ 16,118
Interest cost	18,635	19,251	16,678
Net actuarial (gain) loss	(8,154)	29,738
Benefits paid	(18,084)	(17,606)
Benefit obligation, end of year	462,819	456,911	412,608
Fair value of plan assets: at beginning of year	438,708	437,829
Actual return on plan assets	(14,421)	17,826
Employer contributions	86,123	1,123
Benefit payments	(18,084)	(17,606)
Expenses	(776)	(464)
Fair value of plan assets, end of year	491,550	438,708	437,829
Funded status at end of year-net asset (liability)				28,731	(18,203)
Unrecognized loss: at beginning of year	72,858	28,285
Net actuarial loss (gain)	36,707	44,573
Unrecognized loss at end of year	109,565	72,858	28,285
Projected benefit obligation	456,911	412,608	412,608	462,819	456,911
Accumulated benefit obligation				429,338	426,073
Fair value of plan assets	438,708	437,829	437,829	491,550	438,708
Other Post-retirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation: at beginning of year	28,368	31,592
Service cost	117	126	215
Interest cost	891	1,140	1,317
Net actuarial (gain) loss	(5,905)	(3,467)
Plan participants' contributions	1,334	1,300
Benefits paid	(2,524)	(2,323)
Benefit obligation, end of year	22,281	28,368	31,592
Employer contributions	1,190	1,023
Plan participants' contributions	1,334	1,300
Benefit payments	(2,524)	(2,323)
Funded status at end of year-net asset (liability)				(22,281)	(28,368)
Unrecognized loss: at beginning of year	3,358	7,189
Net actuarial loss (gain)	(5,911)	(3,831)
Unrecognized loss at end of year	(2,553)	3,358	7,189
Projected benefit obligation	$ 28,368	$ 31,592	$ 31,592	$ 22,281	$ 28,368