Recent Accounting Pronouncements (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Jan. 01, 2020	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Fixed maturity securities, allowance for credit loss	$ 67,463
Cumulative effect adjustment resulting from changes in accounting principles	5,537,544		$ 6,118,342
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	7,877,371		7,932,372	$ 7,721,039	$ 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			(30,514)		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	$ (109,514)		124,514	$ 34,236	(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			$ 24,952		$ 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