New Accounting Pronouncements (Details) - USD ($) $ in Thousands		9 Months Ended
	Jan. 01, 2020	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Retained earnings		$ 2,068,714		$ 2,014,570
Valuation allowance for mortgage loans	$ 1,200	2,357	$ 2,227	675	$ 675	$ 675	$ 675
Debt securities held to maturity, allowance for expected credit losses	$ 3,300	$ 5,086	4,940	0
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201613Member	us-gaap:AccountingStandardsUpdate201613Member
Cumulative Effect, Period Of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Retained earnings	$ 3,000
Valuation allowance for mortgage loans			0	504		$ 0	$ 0
Debt securities held to maturity, allowance for expected credit losses			$ 0	$ 3,334