Significant Accounting Policies and Pronouncements (Cumulative-effect Adjustment Upon Adoption) (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Other invested assets	$ 377,429		$ 263,074
Total assets	164,680,784		172,425,340
Income taxes payable	0		36,323
Total liabilities	161,947,553		169,590,092
Accumulated other comprehensive income (loss)	(28,296)		165,346
Retained earnings	1,612,435		1,526,310
Total liabilities and equity	$ 164,680,784		172,425,340
Restatement adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total assets			(34,236)
Income taxes payable			3,883
Total liabilities			(48,848)
Retained earnings			14,612
Total liabilities and equity			$ (34,236)
Restatement adjustment | ASU 2016-01
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Other invested assets		$ 8,097
Total assets		8,097
Income taxes payable		1,700
Total liabilities		1,700
Accumulated other comprehensive income (loss)		(1,539)
Retained earnings		7,936
Total equity		6,397
Total liabilities and equity		$ 8,097