Retirement Benefit Plans (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Defined Contribution Plans [Abstract]
Defined contribution plan, recognized	$ 8,625	$ 8,445	$ 8,267
Matching Obligations in Connection with Plans Which are Funded in Cash and Typically Contributed to Plans [Abstract]
Employer contributions during the period	300	300	500
Employee Stock Ownership Plan (ESOP) Disclosures [Line Items]
Provision for expense in connection with ESOP	2,000	2,200	1,800
SERP Defined Benefit Plan [Member]
Employee Stock Ownership Plan (ESOP) Disclosures [Line Items]
Settlement loss	1,500
Benefits paid	7,600
Benefit obligation outstanding	0
Net periodic benefit cost	$ 0	$ 2,500	$ 800
Employee Stock Ownership Plan and Trust (ESOP) [Member]
Employee Stock Ownership Plan (ESOP) Disclosures [Line Items]
Additional shares contributed to ESOP (in shares)	59,200
Shares released from trust (in shares)	59,200
Total remaining balance of shares in the ESOP (in shares)	200