Significant Accounting Policies (Details) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) numberOfSegments	Mar. 31, 2020 USD ($)	Jan. 01, 2020 USD ($)	Dec. 31, 2019 USD ($)		Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Accounting Policies [Abstract]
Number of reportable segments | numberOfSegments			4
Assets:
Other long-term investments	$ 432,325		$ 432,325			$ 326,347
Agent debit balances	428,000		428,000			423,877
Agent debit balances, allowance for credit losses	$ 1,200		$ 1,200			1,000
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201613Member	us-gaap:AccountingStandardsUpdate201602Member	us-gaap:AccountingStandardsUpdate201613Member
Shareholders' equity	$ 7,848,369	$ 6,700,398	$ 7,848,369	$ 6,520,282		7,294,307		$ 6,043,426	$ 5,415,177
Commercial mortgage participations
Assets:
Other long-term investments	164,420		$ 164,420			137,692
Loan receivable, typical term			3 years
Loan receivable, extension term			2 years
Carrying value, net of valuation allowance	164,420		$ 164,420			137,692
Cumulative Effect, Period Of Adoption, Adjustment
Assets:
Agent debit balances					$ (240)
Shareholders' equity						(454)	[1]	105	(497)	[2]
Cumulative Effect, Period Of Adoption, Adjustment | Commercial mortgage participations
Assets:
Other long-term investments					(335)
Cumulative Effect, Period Of Adoption, Adjusted Balance
Assets:
Agent debit balances					423,637
Shareholders' equity						7,293,853			5,414,680
Cumulative Effect, Period Of Adoption, Adjusted Balance | Commercial mortgage participations
Assets:
Other long-term investments					$ 137,357
Retained Earnings
Assets:
Shareholders' equity	$ 5,838,930	$ 5,514,517	$ 5,838,930	$ 5,686,431		5,551,329		5,364,820	5,213,468
Retained Earnings | Cumulative Effect, Period Of Adoption, Adjustment
Assets:
Shareholders' equity						(454)	[1]	$ 105	(497)	[2]
Retained Earnings | Cumulative Effect, Period Of Adoption, Adjustment | Accounting Standards Update 2016-13, Adjustment For Commercial Loan Participations [Member]
Assets:
Shareholders' equity						(265)
Retained Earnings | Cumulative Effect, Period Of Adoption, Adjustment | Accounting Standards Update 2016-13, Adjustment For Agent Debit Balances [Member]
Assets:
Shareholders' equity						$ (189)
Retained Earnings | Cumulative Effect, Period Of Adoption, Adjusted Balance
Assets:
Shareholders' equity									$ 5,212,971

[1] Adoption of Accounting Standard Update (ASU) 2016-13, Financial Instruments—Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments, on January 1, 2020. See more information in Note 2—New Accounting Standards.

[2] Adoption of ASU 2016-02, Leases (Topic 842), on January 1, 2019.