Significant Accounting Policies and Pronouncements (Cumulative-effect Adjustment Upon Adoption) (Details) - USD ($) $ in Thousands		Mar. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Other invested assets	[1]	$ 304,417		$ 263,074
Total assets		169,396,445		172,459,576
Income taxes		786		32,440
Total liabilities		166,669,070		169,638,940
Accumulated other comprehensive income (loss)		78,932		165,346
Retained earnings		1,499,351		1,511,698
Total liabilities and equity		$ 169,396,445		$ 172,459,576
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