Employee Investment Plans (Details) - Subsidiaries [Member] $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Defined Contribution Plan
Employer matching contribution, percent	1.00%	4.50%
Maximum annual contribution per employee, percent		6.00%
Employer discretionary contribution amount		$ 20.7	$ 19.2	$ 17.2
Defined Contribution Plan, Vesting Years		3
First 3 Percent
Defined Contribution Plan
Employer matching contribution, percent		100.00%
Maximum annual contribution per employee, percent		3.00%
Second 3 Percent
Defined Contribution Plan
Employer matching contribution, percent		50.00%
Maximum annual contribution per employee, percent		3.00%
Cash Balance Formula
Defined Contribution Plan
Employer matching contribution, percent		100.00%
Maximum annual contribution per employee, percent		6.00%
Employer additional contribution of base pay, percentage		1.00%
Final Average Earnings Formula
Defined Contribution Plan
Employer matching contribution, percent		55.00%
Maximum annual contribution per employee, percent		6.00%
employer contribution [Member]
Defined Contribution Plan
Employer matching contribution, percent		4.00%