Income Taxes - Narrative (Details) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax [Line Items]
Federal Statutory Tax Rate										35.00%	35.00%	35.00%
Tax Cut and Jobs Act of 2017 benefit to income tax expense										$ 98,500,000
Income taxes	$ (60,354,000)	$ 33,565,000	$ 33,282,000	$ 22,772,000	$ 30,191,000	$ 30,400,000	$ 32,554,000	$ 25,036,000		29,265,000	$ 118,181,000	$ 101,110,000
Deferred tax assets, operating loss carryforwards, state and local	12,200,000									12,200,000
Unrecognized tax benefits that would impact effective tax rate	13,400,000				11,700,000					13,400,000	11,700,000
Unrecognized tax benefits, income tax penalties and interest accrued	1,900,000				$ 2,100,000					1,900,000	2,100,000
Unrecognized tax benefits, interest benefit										300,000	$ 300,000	$ 300,000
Unrecognized tax penalties										0
Canada [Member]
Income Tax [Line Items]
Tax Cuts and Jobs Act of 2017, incomplete accounting, transition tax for accumulated foreign earnings, provisional income tax expense										3,000,000
Foreign [Member]
Income Tax [Line Items]
Tax credits	$ 40,400,000									40,400,000
AFS Investments [Member]
Income Tax [Line Items]
Income taxes										$ 7,800,000
Scenario, Forecast [Member]
Income Tax [Line Items]
Federal Statutory Tax Rate									21.00%
Maximum [Member]
Income Tax [Line Items]
Provisional information accounting incomplete measurement period										1 year