Retirement Benefit Plans - Components of Net Periodic Benefit Cost (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Pension Benefits [Member]
Defined Benefit Plan Net Periodic Benefit Cost [Line Items]
Service cost	$ 12,920	$ 16,118	$ 12,989
Interest cost	19,251	16,678	17,206
Expected return on plan assets	(32,222)	(27,928)	(25,398)
Amortization of net loss/ prior service cost	26	6,570	6,582
Recognized net amortization and deferral	0	0	0
Net periodic benefit cost	(25)	11,438	11,379
Net (loss)/gain recognized during the year	(26)	(6,570)	(6,582)
Net actuarial loss/(gain)	44,599	(76,939)	(7,001)
Total recognized in other comprehensive income	44,573	(83,509)	(13,583)
Total recognized in net periodic benefit cost and other comprehensive income	44,548	(72,071)	(2,204)
Discount rate for benefit obligations	4.11%	4.73%	3.82%
Discount rate for net periodic benefit cost	4.73%	3.82%	4.32%
Expected long-term return on plan assets	7.50%	7.50%	7.50%
Rate of compensation increase		4.00%	3.73%
Other Post-retirement Benefits [Member]
Defined Benefit Plan Net Periodic Benefit Cost [Line Items]
Service cost	126	215	192
Interest cost	1,140	1,317	1,337
Amortization of net loss/ prior service cost	364	1,761	918
Recognized net amortization and deferral	0	0	0
Net periodic benefit cost	1,630	3,293	2,447
Net (loss)/gain recognized during the year	(364)	(1,761)	(966)
Net actuarial loss/(gain)	(3,467)	(5,563)	4,435
Amortization of prior service cost			48
Total recognized in other comprehensive income	(3,831)	(7,324)	3,517
Total recognized in net periodic benefit cost and other comprehensive income	$ (2,201)	$ (4,031)	$ 5,964
Discount rate for benefit obligations	4.02%	4.58%	3.69%
Discount rate for net periodic benefit cost	4.58%	3.69%	4.18%