Employee Benefits (Defined contribution plans) (Details1 )	12 Months Ended
	Dec. 31, 2017 USD ($) components	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Defined Contribution Plans
Number of company contribution components, Retirement Savings Plan | components	3
Fixed percentage of employer's contribution based on eligible compensation	3.00%
Employee contribution limit per calendar year (as a percent of compensation)	50.00%
Employee's contribution matched by employer (in dollars)	$ 1.00
Employer match of employee contributions upto 2% of eligible compensation	$ 1.25
Percentage of eligible compensation, matched $1.25 for by employer for each dollar amount contributed by participants	2.00%
Employer match of employee contributions over 2% upto 5% of eligible compensation	$ 0.50
Percentage of eligible compensation, matched $0.50 for by employer for each dollar amount contributed by participants, low end of range	2.00%
Percentage of eligible compensation, matched $0.50 for by employer for each dollar amount contributed by participants, high end of range	5.00%
Total expense for all components of the company's defined contribution plans	$ 13,500,000	$ 12,800,000	$ 12,000,000