Significant Accounting Policies and Pronouncements (Cumulative-effect Adjustment Upon Adoption) (Details) - USD ($) $ in Thousands		Jun. 30, 2018	Jan. 01, 2018	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Other invested assets	[1]	$ 313,920		$ 263,074
Total assets		169,832,225		172,425,340
Income taxes		11,546		36,323
Total liabilities		167,122,244		169,590,092
Accumulated other comprehensive income (loss)		9,675		165,346
Retained earnings		1,551,327		1,526,310
Total liabilities and equity		$ 169,832,225		172,425,340
Restatement Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total assets				(34,236)
Income taxes				3,883
Total liabilities				(48,848)
Retained earnings				14,612
Total liabilities and equity				$ (34,236)
ASU 2016-01 | Restatement Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Other invested assets			$ 8,097
Total assets			8,097
Income taxes			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

[1] Prior period amounts have been reclassified to conform to current period presentation. See "Adoption of ASU 2016-01" in Note 2 for details