Employee Benefit Plans: (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Defined Benefit Plan Disclosure [Line Items]
Defined Contribution Plan, Employer Matching Contribution, Percent of Match			4.00%
Defined Contribution Plan, Employer Matching Contribution, Percent of Employees' Gross Pay			5.00%
Company's 401(k) Employer Match Expense	$ 6.4	$ 6.5	$ 13.7	$ 13.5
Defined Contribution Plan, Matching Employer Contribution, Cash			12.6
Defined Contribution Plan, Contributions by Employer, Cash			25.7	26.4
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Expected annual employer contributions	52.8		52.8
Required quarterly employer contribution - cash			3.4	$ 6.4
Other Pension, Postretirement and Supplemental Plans
Defined Benefit Plan Disclosure [Line Items]
Expected annual employer contributions	$ 0.9		$ 0.9