Significant Accounting Policies (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Mar. 31, 2020 USD ($)	Mar. 31, 2019 USD ($)	Sep. 30, 2020 USD ($) numberOfSegments	Jun. 30, 2020 USD ($)	Dec. 31, 2019 USD ($)		Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)
Accounting Policies [Abstract]
Number of reportable segments | numberOfSegments			4
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201613Member	us-gaap:AccountingStandardsUpdate201602Member	us-gaap:AccountingStandardsUpdate201613Member
Shareholders' equity	$ 6,520,282	$ 6,043,426	$ 8,224,908	$ 7,848,369	$ 7,294,307		$ 7,312,531	$ 6,700,398	$ 5,415,177
Assets:
Other long-term investments			492,884		326,347
Agent debit balances			438,000		423,877
Accrued interest receivable			457
Agent debit balances, allowance for credit losses			1,200		1,000
Unfunded loan commitment
Assets:
Carrying value, net of valuation allowance			81,000
Commercial mortgage participations
Assets:
Other long-term investments			$ 164,902		137,692
Loan receivable, typical term			3 years
Loan receivable, extension term			2 years
Carrying value, net of valuation allowance			$ 164,902		137,692
Cumulative Effect, Period Of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Shareholders' equity		105			(454)	[1]			(497)	[2]
Assets:
Agent debit balances					(240)
Cumulative Effect, Period Of Adoption, Adjustment | Commercial mortgage participations
Assets:
Other long-term investments					(335)
Cumulative Effect, Period Of Adoption, Adjusted Balance
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Shareholders' equity					7,293,853				5,414,680
Assets:
Agent debit balances					423,637
Cumulative Effect, Period Of Adoption, Adjusted Balance | Commercial mortgage participations
Assets:
Other long-term investments					137,357
Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Shareholders' equity	$ 5,686,431	5,364,820	$ 6,002,407	$ 5,838,930	5,551,329		$ 5,686,636	$ 5,514,517	5,213,468
Retained Earnings | Cumulative Effect, Period Of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Shareholders' equity		$ 105			(454)	[1]			(497)	[2]
Retained Earnings | Cumulative Effect, Period Of Adoption, Adjustment | Accounting Standards Update 2016-13, Adjustment for Commercial Loan Participations
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Shareholders' equity					(265)
Retained Earnings | Cumulative Effect, Period Of Adoption, Adjustment | Accounting Standards Update 2016-13, Adjustment for Agent Debit Balances
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Shareholders' equity					(189)
Retained Earnings | Cumulative Effect, Period Of Adoption, Adjusted Balance
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Shareholders' equity					$ 5,550,875				$ 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.