Employee and Non-Employee Director Stock and Benefit Plans - Retirement Savings Plan (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Employee and Non Employee Director Benefit Plans
Director benefit plan, shares vested and deferred	$ 20,485
Retirement Savings Plan (RSP)
Employee and Non Employee Director Benefit Plans
In accordance with the Internal Revenue Code, the maximum annual eligible compensation under the plan, prior to January 1, 2011	$ 270,000
Company matching contribution, under the plan (as a percent)	50.00%
Maximum eligible compensation (as a percent)	4.00%
Benefit costs	$ 4,300,000	$ 4,400,000	$ 4,900,000
Directors' Retainer Fee Subplan
Employee and Non Employee Director Benefit Plans
Annual retainer fee received by non-employee directors, excluding the lead director and chairman, since 2006	70,000
Annual retainer fee received by lead director and chairman since August, 2008	$ 105,000
Deferred shares or units received	11,115	12,363	13,846
Deferred shares or units outstanding	130,940
Benefit costs	$ 800,000	$ 800,000	$ 1,200,000