Recent Accounting Pronouncements and Accounting Policies (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Jun. 30, 2020	Jan. 01, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect adjustment resulting from changes in accounting principles	$ 6,039,672			$ 6,118,342
Fixed maturity securities, allowance for credit loss	2,567
Cumulative effect adjustment resulting from changes in accounting principles | Accounting Standards Update 2016-13
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Premium receivable and reinsurance recoverable, allowance for credit loss			$ 5,700
Fixed maturity securities, allowance for credit loss			24,800
Retained earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect adjustment resulting from changes in accounting principles	8,057,556	$ 7,927,280		7,932,372	$ 7,971,019	$ 7,826,015	$ 7,558,619
Retained earnings | Cumulative effect adjustment resulting from changes in accounting principles
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect adjustment resulting from changes in accounting principles		0		(30,514)		0	0
Retained earnings | Cumulative effect adjustment resulting from changes in accounting principles | Accounting Standards Update 2016-13
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect adjustment resulting from changes in accounting principles			(5,700)
Accumulated Other Comprehensive Income (Loss)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect adjustment resulting from changes in accounting principles	$ 247,480	209,210		124,514	$ 128,293	152,915	(91,491)
Accumulated Other Comprehensive Income (Loss) | Cumulative effect adjustment resulting from changes in accounting principles
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect adjustment resulting from changes in accounting principles		$ 0		$ 24,952		$ 0	$ 0
Investments | Cumulative effect adjustment resulting from changes in accounting principles | Accounting Standards Update 2016-13
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect adjustment resulting from changes in accounting principles			200
Investments | Retained earnings | Cumulative effect adjustment resulting from changes in accounting principles | Accounting Standards Update 2016-13
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect adjustment resulting from changes in accounting principles			(24,800)
Investments | Accumulated Other Comprehensive Income (Loss) | Cumulative effect adjustment resulting from changes in accounting principles | Accounting Standards Update 2016-13
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect adjustment resulting from changes in accounting principles			$ 25,000