Condensed Consolidated Statements of Changes in Stockholders' Equity - USD ($) $ in Thousands	Total	Preferred stock	Common stock	Treasury Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income (Loss)	Retained Earnings	Non-controlling Interest
Total stockholders' equity, amount, beginning balance at Dec. 31, 2018	$ 2,200,871	$ 450,000	$ 1,129		$ 1,057,783	$ (52,130)	$ 764,056	$ (19,967)
Increase (Decrease) in Stockholders' Equity
Net income (loss)	162,397						169,634	(7,237)
Foreign currency translation adjustment, net of tax	(3,198)					(3,198)
Change in unrealized gain (loss) on investments, net of tax	124,048					115,238		8,810
Common stock dividends declared	(9,049)						(9,049)
Preferred stock dividends declared	(16,800)						(16,800)
Common stock issued under employee stock plans and exercises of stock options, value	247		3		244
Shares withheld related to net share settlement, value	(3,140)				(3,140)
Stock-based compensation	5,492				5,492
Total stockholders' equity, amount, ending balance at Jun. 30, 2019	2,460,868	450,000	1,132		1,060,379	59,910	907,841	(18,394)
Total stockholders' equity, amount, beginning balance at Mar. 31, 2019	2,331,482	450,000	1,131		1,058,061	581	843,415	(21,706)
Increase (Decrease) in Stockholders' Equity
Net income (loss)	77,058						77,876	(818)
Foreign currency translation adjustment, net of tax	166					166
Change in unrealized gain (loss) on investments, net of tax	63,293					59,163		4,130
Common stock dividends declared	(4,525)						(4,525)
Preferred stock dividends declared	(8,925)						(8,925)
Common stock issued under employee stock plans and exercises of stock options, value	154		1		153
Shares withheld related to net share settlement, value	(705)				(705)
Stock-based compensation	2,870				2,870
Total stockholders' equity, amount, ending balance at Jun. 30, 2019	2,460,868	450,000	1,132		1,060,379	59,910	907,841	(18,394)
Increase (Decrease) in Stockholders' Equity
Cumulative Effect of New Accounting Principle in Period of Adoption	(1,010)
Cumulative Effect of New Accounting Principle in Period of Adoption | Accounting Standards Update 2016-13	(147)						(863)
Total stockholders' equity, amount, beginning balance at Dec. 31, 2019	2,617,494	450,000	1,134	$ 0	1,065,634	74,548	1,058,138	(31,960)
Increase (Decrease) in Stockholders' Equity
Net income (loss)	275,186						267,333	7,853
Foreign currency translation adjustment, net of tax	(484)					(484)
Change in unrealized gain (loss) on investments, net of tax	121,606					112,800		8,806
Common stock repurchased	(8,482)			(8,482)
Common stock dividends declared	(11,357)						(11,357)
Preferred stock dividends declared	(16,800)						(16,800)
Common stock issued under employee stock plans and exercises of stock options, value	836		5		831
Shares withheld related to net share settlement, value	(3,491)				(3,491)
Stock-based compensation	6,178				6,178
Total stockholders' equity, amount, ending balance at Jun. 30, 2020	2,979,676	450,000	1,139	(8,482)	1,069,152	186,864	1,296,451	(15,448)
Total stockholders' equity, amount, beginning balance at Mar. 31, 2020	2,671,488	450,000	1,137	0	1,066,075	43,845	1,144,473	(34,042)
Increase (Decrease) in Stockholders' Equity
Net income (loss)	174,613						166,574	8,039
Foreign currency translation adjustment, net of tax	(214)					(214)
Change in unrealized gain (loss) on investments, net of tax	153,788					143,233		10,555
Common stock repurchased	(8,482)			(8,482)
Common stock dividends declared	(5,671)						(5,671)
Preferred stock dividends declared	(8,925)						(8,925)
Common stock issued under employee stock plans and exercises of stock options, value	448		2		446
Shares withheld related to net share settlement, value	(385)				(385)
Stock-based compensation	3,016				3,016
Total stockholders' equity, amount, ending balance at Jun. 30, 2020	$ 2,979,676	$ 450,000	$ 1,139	$ (8,482)	$ 1,069,152	$ 186,864	$ 1,296,451	$ (15,448)