New Accounting Pronouncements (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Jan. 01, 2020	Dec. 31, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Retained earnings	$ 2,057,889			$ 2,014,570
Valuation allowance for mortgage loans	2,227	$ 1,885	$ 1,200	675	$ 675	$ 675	$ 675
Debt securities held to maturity, allowance for expected credit losses	$ 4,940	6,039	3,300	0
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201613Member
Cumulative effect, 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