Employee Benefit Plans (Narrative) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2018 USD ($) funds fund	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Defined Benefit Plan Disclosure [Line Items]
Expenses relating to defined contribution plans	$ 41.8	$ 36.7	$ 30.1
Consecutive number of years	3 years
Number of years preceding retirement	10 years
Amortization of the net actuarial pension loss included as a component of accumulated other comprehensive income	$ 2.7
Percentage of equity securities invested in UK companies	30.00%
Percentage of equity securities invested in other markets	70.00%
Number of fixed maturity mutual funds | funds	5
Accumulated benefit obligation	$ 168.1	$ 195.1
Expected plan contributions in 2019	3.4
Expected benefits to be paid in 2019	3.8
Expected benefits to be paid in 2020	3.8
Expected benefits to be paid in 2021	3.9
Expected benefits to be paid in 2022	4.0
Expected benefits to be paid in 2023	4.1
Aggregate benefits expected to be paid from 2024 to 2028	$ 22.0
United Kingdom [Member] | Foreign Governments [Member]
Defined Benefit Plan Disclosure [Line Items]
Number of fixed maturity mutual funds | funds	2
United Kingdom [Member] | Corporate Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Number of fixed maturity mutual funds | funds	1
Other Foreign Governments [Member] | Foreign Governments [Member]
Defined Benefit Plan Disclosure [Line Items]
Number of fixed maturity mutual funds | funds	1
United Kingdom And Other Foreign Governments [Member] | Corporate Bond Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Number of fixed maturity mutual funds | fund	1
Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Target asset allocation, percent	47.00%
Actual asset allocation, percent	45.00%	46.00%
Fixed Maturities [Member]
Defined Benefit Plan Disclosure [Line Items]
Target asset allocation, percent	53.00%
Actual asset allocation, percent	55.00%	54.00%