Fair Values of Financial Instruments - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Maturity of corporate debt with similar credit risk	20 years
US treasury bond maturity	20 years
Discounted cash flow, discount rate	9.07%	8.67%	9.41%
Discounted cash flow, discount rate premium	1.00%
Institutional Products Group
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Discounted cash flow sensitivity analysis discount rate rncrease	1.00%
Europe
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Discounted cash flow sensitivity analysis discount rate rncrease	1.00%
Foreign exchange forward
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gain (Loss)	$ (840,000)	$ 2,573,000	$ 1,112,000