Postretirement Benefits - Activity for the Active SERP (Details) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Compensation and Retirement Disclosure [Abstract]
Premiums paid for insurance coverage	$ 2,050,000	$ 10,068,000	$ 2,150,000
Company owned life insurance	37,267,000	34,178,000
Exchange Traded Funds	48,999,000	45,014,000
Total investments of SERP	86,266,000	79,192,000
Liability for SERP	74,687,000	67,243,000
Cash deposited into investment fund	$ 0	$ 0	$ 0