Investments (Unrealized Losses) (Details) $ in Thousands		3 Months Ended		9 Months Ended
	Jan. 01, 2020 USD ($)	Sep. 30, 2020 USD ($) security	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) security	Sep. 30, 2019 USD ($)	Jun. 30, 2020 USD ($)	Dec. 31, 2019 USD ($)	Jun. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)
Schedule of Investments [Line Items]
Securities, other-than-temporarily impaired			$ 1,900		$ 7,800
Valuation allowance for mortgage loans	$ 1,200	$ 2,357	$ 675	$ 2,357	675	$ 2,227	$ 675	$ 675	$ 675
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201613Member			us-gaap:AccountingStandardsUpdate201613Member
Decrease to retained earnings		(2,068,714)		$ (2,068,714)			(2,014,570)
Other long-term investments		78,579		78,579			62,090
Real Estate
Schedule of Investments [Line Items]
Other long-term investments		$ 33,900		33,900			34,600
Operating income from real estate				$ 2,200	$ 2,100
Cumulative Effect, Period Of Adoption, Adjustment
Schedule of Investments [Line Items]
Valuation allowance for mortgage loans						$ 0	$ 504	$ 0	$ 0
Decrease to retained earnings	$ (3,000)
Cumulative Effect, Period Of Adoption, Adjustment | Accounting Standards Update 2016-13
Schedule of Investments [Line Items]
Decrease to retained earnings	$ (400)
Debt Securities
Schedule of Investments [Line Items]
Gross unrealized losses, number of issues | security		68		68
Gross unrealized losses, percentage of total debt		5.00%		5.00%
Gross unrealized losses, market value as a percent of amortized cost		93.30%		93.30%
Gross unrealized losses, number of securities with maturities of 12 months or greater | security		8		8
Gross unrealized losses, number of securities with maturities of 12 months or greater (as a percentage)		11.80%		11.80%
Debt Securities | External Credit Rating, Investment Grade
Schedule of Investments [Line Items]
Gross unrealized losses, number of securities rated investment grade | security		45		45
Travis County
Schedule of Investments [Line Items]
Gain on sale of property		$ 2,700